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7 




E3 



S^^^^^^^^^^^^^3S 



Marine Biological Laboratory Library 

Woods Hole, Mass. 




Presented by 

Hillary House 
Publishers Ltd, 
Jan. 8, 1962 




s^^^^^^^^^^^^^s 




n 



The Morpholog)' of 
Pteridophytes 



BIOLOGICAL SCIENCES 



PROFESSOR H. MUNRO FOX 

M.A., F.R.S. 

Emeritus Professor of Zoology in the University 

of London 



THE MORPHOLOGY 
OF PTERIDOPHYTES 

The structure of ferns and allied plants 



K. R. SPORNE 

M.A., PH.D., F.L.S. 

Fellow of Downing College, Cambridge 
and University Lecturer in Botany 




(^i\ 



HUT HILLARY HOUSE RARY 

New York 



HUTCHINSON & CO. (Publishers) LTD 
178-202 Great Portland Street, London, W.i 

London Melbourne Sydney 

Auckland Bombay Toronto 

Johannesburg New York 

• 

First published 1962 



© K. R. Sporne 1962 

This book has been set in Times New Roman type 
face. It has been printed in Great Britain by The 
Anchor Press, Ltd., in Tiptree, Essex, on Antique 

Wove paper. 



To 
H. H. T. 

in grateful memory 



Contents 



3 
4 

5 
6 

7 



Preface 

Introduction 

Psilophytopsida 

Psilotopsida 

Lycopsida 

Sphenopsida 

Pteropsida 

General Conclusions 

Bibliography 



Index 



9 
II 

28 

38 
50 

94 

114 

175 
183 
189 




807^18 



Preface 



For many years morphology was regarded as a basic discip- 
line in the study of botany and, consequently, there have 
been many textbooks dealing with the subject. The pterido- 
phytes occupied varying proportions of these, and there 
were even some textbooks devoted to a single group, such as 
the ferns, within the pteridophytes. However, to the best of 
my knowledge, no book dealing solely with the pteridophytes 
has been pubHshed in the western hemisphere since 1936. 
Some of the old classics have recently been reprinted, but 
there is a need for a reappraisal of the old theories in the 
Ught of recent knowledge. Contrary to general behef, the 
study of morphology is a very live one, and many important 
advances have been made, on both sides of the Atlantic, in 
the last decade. Exciting new fossils have been discovered 
and new techniques have been developed for studying living 
organisms, to say nothing of the discovery of an entirely new 
genus of lycopods in the High Andes of Peru. 

Naturally, a book of this kind owes much to those that 
have gone before, and the most important of these are Hsted 
in the bibhography. The majority of the illustrations have 
been redrawn from pubHshed accounts, either in these text- 
books or in research literature, and this fact has been 
acknowledged in every case by reference to the author's 
name. 

I should Hke, also, to acknowledge the help given un- 
consciously by my colleagues in the Botany School, Cam- 
bridge, discussions with whom over the years have crystal- 

9 



10 PREFACE 

lized many of the ideas incorporated in this book. Most of 
all, I owe a debt of gratitude to my teacher and friend, the 
late Hugh Hamshaw Thomas, sc.d., f.r.s., who guided my 
first thoughts on the evolution of plants and who was a 
constant source of inspiration for more than twenty-five 
years. It was he who first demonstrated to me that the study 
of Hving plants is inseparable from that of fossils, a fact 
which forms the basis for the arrangement of this book, in 
which hving and fossil plants are given equal importance. 

Finally, my grateful thanks are due to my wife for her 
helpful criticisms during the preparation of the manuscript. 

Jv. rv. S. 

Cambridge 



Introduction 



The study of the morphology of Uving organisms is one of 
the oldest branches of science, for it has occupied the 
thoughts of man for at least 2,500 years. Indeed, the very 
word 'morphology' comes from the ancient Greeks, while 
the names of Aristotle and Theophrastus occupy places of 
importance among the most famous plant morphologists. 
Strictly translated, morphology means no more than the 
study of form, or structure. One may well ask, therefore, 
wherein lies the intense fascination that has captured the 
thoughts and imagination of so many generations of 
botanists from Aristotle's time to the present day; for the 
study of structure alone would be dull indeed. The answer is 
that, over the centuries, morphology has come to have wider 
imphcations, as Arber- has explained in her Natural Philo- 
sophy of Plant Form. In this book she points out that the 
purpose of the morphologists is to 'connect into one 
coherent whole all that may be held to belong to the intrinsic 
nature of a living being'. This involves the study, not only of 
structures as such, but also of their relations to one another 
and their co-ordination throughout the hfe of the organism. 
Thus, morphology impinges on all other aspects of hving 
organisms (physiology, biochemistry, genetics, ecology, etc.). 
Furthermore, the morphologist must see each hving organ- 
ism in its relationship to other living organisms (taxonomy) 
and to extinct plants (paleobotany) whose remains are 
known from the fossil record of past ages extending back in 

II 



12 THE MORPHOLOGY OF PTERIDOPHYTES 

time certainly 500 million years and probably as far back as 
1,000 million (some even say 2,000 million) years. Clearly, 
the morphologist cannot afford to be a narrow specialist. 
He must be a biologist in the widest possible sense. 

From taxonomy and paleobotany, the plant morpholo- 
gist is led naturally to the consideration of the course of 
evolution of plants (phylogeny), which to many botanists 
has the greatest fascination of all. However, it must be 
emphasized that here the morphologist is in the greatest 
danger of bringing discredit on his subject. His theories are 
not capable of verification by planned experiments and 
cannot, therefore, be proved right or wrong. At the best, 
they can be judged probable or improbable. Theories 
accepted fifty years ago may have to be abandoned as im- 
probable today, now that more is known of the fossil record, 
and, hkewise, theories that are acceptable today may have 
to be modified or abandoned tomorrow. It is essential, there- 
fore, that the morphologist should avoid becoming dog- 
matic if he is ever to arrive at a true understanding of the 
course of evolution of hving organisms. 

Within the plant kingdom the range of size is enormous, 
for, on the one hand, there are unicellular algae and bacteria 
so small that individuals are visible only under the micro- 
scope, while, on the other hand, there are seed-bearing 
plants, such as the giant Redwoods of California and the 
Gums of AustraUa, some of which are probably the largest 
living organisms that the world has ever known. Accompany- 
ing this range of size, there is a corresponding range of 
complexity of internal anatomy and of life-history. Some- 
where between the two extremes, both in structure and in 
Hfe-cycle, come the group of plants known as Pteridophytes, 
for they share with seed plants the possession of well- 
developed conducting tissues, xylem and phloem, but differ 
from them in lacking the seed habit. Internally, they are 
more complex than mosses and Uverworts, yet in life-cycle 
they differ from them only in matters of degree. 



INTRODUCTION 



13 



The basic life-cycle, common to bryophytes and pterido- 
phytes, is represented diagrammatically in Fig. i. Under 
normal circumstances there is a regular alternation between 
a gametophyte (sexual) phase and a sporophyte (asexual) 
phase. The male gametes, produced in numbers from an- 
theridia, are known as antherozoids, since they are flagel- 
lated and are able to swim in water, while the female gametes 
(Qgg cells) are non-motile and are borne singly in flask- 
shaped archegonia. Fusion between an egg cell and an 




Gametophyte- 



Egg ("V (E) Spermatozoid 




II 



2n 



Zygote Sporophyte 




++^ ® ^ Spores 



-ri • ^(n) 



Meiosis 



Fig. 1 

Life cycle of a homosporous pteridophyte 

antherozoid results in the formation of a zygote, which 
contains the combined nuclear material of the two gametes. 
Its nucleus contains twice as many chromosomes as either 
of the gamete nuclei and it is therefore described as diploid. 
The zygote develops directly by mitotic divisions into the 
sporophyte which is, Ukewise, diploid. Uhimately, there are 
released from the sporophyte a number of non-motile spores, 
in the formation of which meiosis brings about a reduction of 
the nuclear content to the haploid number of chromosomes. 
The life-cycle is then completed when these spores germinate 
and grow, by mitotic divisions, into haploid gametophytes. 
In mosses and liverworts, the dominant phase in the life- 
cycle is the gametophyte, for the sporophyte is retained upon 
it throughout its Hfe and is either partially or completely 



14 THE MORPHOLOGY OF PTERIDOPHYTES 

dependent on it for nutrition. By contrast, among pterido- 
phytes the sporophyte is the dominant generation, for it very 
soon becomes independent of the gametophyte (prothallus) 
and grows to a much greater size. Along with greater size 
is found a much greater degree of morphological and 
anatomical complexity, for the sporophyte is organized into 
stems, leaves and (except in the most ancient fossil pterido- 
phytes and the most primitive Uving members of the group) 
roots. Only the sporophyte shows any appreciable develop- 
ment of conducting tissues (xylem and phloem), for although 
there are recorded instances of such tissues in gametophytes, 
they are rare and the amounts of xylem and phloem are 
scanty. Furthermore, the aerial parts of the sporophyte are 
enveloped in a cuticle in which there are stomata, giving 
access to complex aerating passages that penetrate between 
the photosynthetic pahsade and mesophyll cells of the leaf. 
All these anatomical complexities confer on the sporo- 
phyte the potentiaUty to exist under a much wider range of 
environmental conditions than the gametophyte. However, 
in many pteridophytes these potentialities cannot be realized, 
for the sporophyte is limited to those habitats in which the 
gametophyte can survive long enough for fertilization to 
take place. This is a severe hmitation on those species whose 
gametophytes are thin plates of cells that lack a cuticle and 
are, therefore, susceptible to dehydration. Not all gameto- 
phytes, however, are Umited in this way, for in some pterido- 
phytes they are subterranean and in others they are retained 
within the resistant wall of the spore and are thus able to 
survive in a much wider range of habitats. It is notable that 
wherever the gametophyte is retained within the spore the 
spores are of different sizes (heterosporous), the larger 
megaspores giving rise to female prothalH which bear only 
archegonia, and the smaller microspores giving rise to male 
prothaUi bearing only antheridia. Why this should be is not 
known with certainty, but two possible reasons come to 
mind, both of which probably operate together. 



INTRODUCTION 15 

The first concerns the nutrition of the prothallus and the 
subsequent embryonic sporophyte. The retention of the 
gametophyte within a resistant spore wall severely Hmits its 
powers of photosynthesis and may even prevent it alto- 
gether. Hence, it is necessary for such a prothallus to be 
provided with abundant food reserves; the larger the spore, 
the more that can be stored within it. This may well account 
for the lage size of the spores which are destined to contain 
an embryo sporophyte, but it does not explain why the pro- 
thalli should be unisexual (dioecious). This is most probably 
concerned with out-breeding. It is widely accepted that any 
plant which habitually undergoes inbreeding is less likely to 
produce new varieties than one which has developed some 
device favouring out-breeding, and that such a plant is at a 
disadvantage in a changing environment. It will tend to lag 
behind in evolution. Now, monoecious gametophytes (bear- 
ing both archegonia and antheridia) are much more hkely to 
be self-fertihzed than cross-fertilized, unless they are actually 
submerged in water. Yet, dioecious prothalli in a terrestrial 
environment would be at an even greater disadvantage, for 
they might never achieve fertilization at all, so long as the 
antherozoid has to bear the whole responsibility of finding 
the archegonium. This is where heterospory may operate to 
the advantage of plants with dioecious prothalli. Those 
spores which are destined to produce male prothaUi need 
not carry large food reserves and can, therefore, afford to be 
reduced in size to the barest minimum. From the same initial 
resources, vast numbers of microspores can be produced and 
this will allow some of the responsibility for reaching an 
archegonium to be transferred to them. Blown by the wind, 
they can travel great distances and some, at least, will fall on 
a female prothallus in close proximity to an archegonium. 
Thus, when the male prothallus develops, the antherozoids 
Uberated from the antheridia have only a short distance to 
swim and, in order to do so, need only a thin film of 
moisture. Under ordinary circumstances, the chances may 



l6 THE MORPHOLOGY OF PTERIDOPHYTES 

be quite small that the particular microspore will have come 
from the same parent sporophyte as the megaspore and thus 
a fair degree of out-breeding will have been achieved. The 
relative emancipation from the aquatic environment pro- 
vided by the heterosporous habit will confer on the sporo- 
phyte the freedom to grow almost anywhere that its own 
potentiahties allow and the possibiUty of out-breeding will 
favour more rapid evolution of those potentiahties. Most 
morphologists agree that the evolution of heterospory was a 
necessary step in the evolution of the seed habit and that, 
therefore, it is one of the most important advances in the 
whole story of land plant evolution. 

The Ufe-cycle of a typical heterosporous pteridophyte may 
be represented diagrammatically as in Fig. 2. 

The distinction between heterospory and homospory is 
one of the criteria used in the classification of pteridophytes, 
in accordance with the general behef that reproductive 
organs are a better guide to phylogenetic relationships than 
are vegetative organs. They are held to be more *con- 
servative', in being less susceptible to the immediate influence 
of the environment. Likewise, therefore, the manner in 
which the sporangia develop and the way in which they 
are borne on the sporophyte constitute important taxo- 
nomic characters. 

The sporangium, in all pteridophytes, is initiated by the 
laying down of a cross-wall in a superficial cell, or group of 
cells. Since this wall is pericUnal (i.e. parallel to the surface) 
each initial cell is divided into an outer and an inner daughter 
cell. If the sporogenous tissue is derived from the inner 
daughter cell, the sporangium is described as *eusporangiate' 
and, if from the outer, as *leptosporangiate'. This definition 
of the two types of sporangium is usually expanded to in- 
clude a number of other differences. Thus, in leptosporangi- 
ate forms, the sporangium wall and the stalk, as well as the 
spores, are derived from the outer daughter cell, but, in 
eusporangiate forms, adjacent cells may become involved 



INTRODUCTION 



17 



in the formation of part of the sporangium wall and the stalk 
(if any). Furthermore, the sporangium is large and massive 
in eusporangiate forms, the wall is several cells thick and the 
spore content is high, whereas, in leptosporangiate forms, the 
sporangium is small, the wall is only one cell thick and the 
spore content is low. Of these two types, the eusporangiate 
is primitive and the leptosporangiate advanced. 




Eggf " ) (^ Spermatozoid 



Zygote 



5 Gametophyte 



cf Gametophyte 




Meiosis 



2n 




I^^@ Microspores 



-H — ► — —(^ 








*~~vIL//'~^ Megaspores 



B 



© 



Sporophyte Meiosis 

Fig. 2 
Life cycle of a heterosporous pteridophyte 



Until the early years of this century, it was widely beheved 
that sporangia could be borne only on leaves and that such 
fertile leaves, known as 'sporophylls', were an essential part 
of all sporophytes. However, the discovery of Devonian 
pteridophytes that were completely without leaves of any 
kind, fertile or sterile, has led most morphologists to 
abandon this 'sporophyll theory'. It is now accepted that in 
some groups sporangia may be borne on stems, either 
associated or not with leaves, and in others actually on the 
leaves. 

Important as reproductive organs are in classification, 



l8 THE MORPHOLOGY OF PTERIDOPHYTES 

vegetative organs are nevertheless of considerable import- 
ance in classifying pteridophytes, for the shape, size, arrange- 
ment and venation of leaves (and even presence or absence 
of leaves) are fundamental criteria. It so happens that it is 
difficult, if not impossible, to devise a definition of the term 
'leaf that is entirely satisfying, but, for practical purposes, 
it may be said that among pteridophytes there are two very 
different types of leaf, known respectively as megaphylls and 
microphylls. The famiUar fern frond is an example of the 
former; it is large, branches many times and has branching 
veins. By contrast, microphylls are relatively small, rarely 
branch and possess either a limited vascular supply or none 
at all ; the leaf trace, if present, is single and either remains 
unbranched within the microphyll or, if it branches at all, it 
does so to a limited degree and in a dichotomous manner. 

As might be expected, the leaf traces supplying micro- 
phylls cause little disturbance when they depart from the 
vascular system (stele) of the parent axis, whereas those 
supplying megaphylls are usually (though not invariably) 
associated with leaf gaps. A stele without leaf gaps is termed 
a protostele, the simplest type of all being the soUd proto- 
stele. Fig. 3A illustrates its appearance diagrammatically as 
seen in transverse section. In the centre is a solid rod of 
xylem which is surrounded by phloem and then by pericycle, 
the whole stele being bounded on the outside by a con- 
tinuous endodermis. Another variety of protostele is the 
medullated protostele, illustrated in Fig. 3B. In this the 
central region of the xylem is replaced by parenchyma. Yet 
other varieties of protostele will be described as they are 
encountered in subsequent chapters. Steles in which there 
are leaf gaps are known as dictyosteles, if the gaps occur 
frequently enough to overlap, and as solenosteles if they are 
more distantly spaced. Fig. 3C is a diagrammatic represen- 
tation of a solenostele as seen in transverse section passing 
through a leaf gap. The most remarkable feature is the way 
in which the inside of the xylem cyhnder is lined with 






___!..... 


I'i'i'i 
.III 

1 1 1 1 

i'i'i'i 







* ^ 


K^'' 


Phloem 


Metaxylem 


Medu 


Ila or pith 



• Protoxylem 

„._.-.— Endodermis 




Fig. 3 

Fem steles: a, solid protostele; b, meduUated protostele; 
c, D, solenostele; e, f, dictyostele; g, dicyclic stele 



20 THE MORPHOLOGY OF PTERIDOPHYTES 

phloem, pericycle and endodermis, as if these tissues had 
'invaded' the central parenchymatous region (though, need- 
less to say, the developmental processes do not involve any 
such invasion). Fig. 3E illustrates the structure of a dictyo- 
stele in which three leaf gaps are visible in the one transverse 
section. Frequently it happens that each leaf gap is associ- 
ated with the departure of several leaf traces to the leaf, but 
in this example, for clarity, only one trace is shown supply- 
ing each leaf. The remaining portions of the stele are 
referred to as meristeles and, although in transverse section 
they appear to be unconnected, when dissected out and viewed 
as three-dimensional objects they are seen to form a network. 
Figs. 3D and 3F are perspective sketches of a solenostele 
and a dictyostele, respectively, from which the surrounding 
cortex and ground tissue have been removed in this way. 

It must be pointed out at this stage that some morpholo- 
gists use a different system of terminology and group to- 
gether the medullated protostele and the solenostele as 
varieties of so-called siphonosteles, on the grounds that 
each has a hollow cylinder of xylem. The former they des- 
cribe as an ectophloic siphonostele, because the phloem is 
restricted to the outside of the xylem, and the latter they 
describe as an amphiphloic siphonostele, because the phloem 
lies both outside and inside the xylem. This practice, how- 
ever, has disadvantages. First, it tends to exaggerate the 
difference between the solenostele and the dictyostele — a 
difference that reflects little more than a difference in the 
direction of growth, for where leaves arise at distant inter- 
vals on a horizontal axis their leaf gaps are unUkely to over- 
lap, whereas leaves on a vertical axis are often so crowded 
that their leaf gaps must overlap. Secondly, it overlooks the 
fundamental distinction between the solenostele and the 
medullated protostele — a physiological distinction depend- 
ing on the position of the endodermis. 

When gaps occur in a stele without any associated leaf 
traces, they are described as perforations and the stele is said 



INTRODUCTION 21 

to be perforated. Thus, there may be perforated solenosteles 
which, at a first glance, might be confused with dictyosteles ; 
however, as soon as attention is paid to the relationship 
between leaf traces and perforations, the distinction becomes 
clear. When more than one stele is visible in any one trans- 
verse section the plant is described as polystelic. Yet another 
variant is the polycyclic stele, in which there are two or more 
co-axial cyUnders of conducting tissue (Fig. 3G). 

All the vascular systems mentioned so far are composed 
entirely of primary tissues, i.e. tissues formed by the matura- 
tion of cells laid down by the main growing point (apical 
meristem). It is customary to draw a rough distinction 
between tissues that differentiate before cell elongation has 
finished and those that differentiate only after such growth 
has ceased. In the former case, the xylem and phloem are 
described as protoxylem and protophloem. They are so con- 
structed that they can still alter their shape and can, thereby, 
accommodate to the continuing elongation of the adjacent 
cells. Accordingly, it is usual for the Ugnification of proto- 
xylem elements to be laid down in the form of a spiral, or 
else in rings. Metaxylem and metaphloem elements, by 
contrast, do not alter their size or shape after differentiation. 

The order in which successive metaxylem elements mature 
may be centripetal or centrifugal. When the first xylem to 
differentiate is on the outside and differentiation proceeds 
progressively towards the centre, the xylem is described as 
exarch and all the metaxylem is centripetal. When the proto- 
xylem is on the inner side of the metaxylem and differentia- 
tion occurs successively away from the centre, the xylem is 
described as endarch and all the metaxylem is centrifugal. 
A third arrangement is known as mesarch, where the proto- 
xylem is neither external nor central and differentiation pro- 
ceeds both centripetally and centrifugally. In Figs. 3A-3C 
the xylem is mesarch, while in Fig. 3G it is endarch. 

In addition to primary vascular tissues, some pterido- 
phytes possess a vascular cambium from which secondary 



22 THE MORPHOLOGY OF PTERIDOPHYTES 

xylem and secondary phloem are formed. Cambial cells 
possess the power of cell division even though the surround- 
ing tissues may have lost it; they may either have retained 
this power throughout the lapse of time since they were laid 
down in the apical meristem, or they may have regained it 
after a period of temporary differentiation. While relatively 
uncommon in living pteridophytes, a vascular cambium was 
widely present in coal-age times, when many members of the 
group grew to the dimensions of trees. Just as, at the present 
day, all trees develop bark on the outside of the trunk and 
branches by the activity of a cork cambium, so also did these 
fossil pteridophytes. In some, the activity of this meristem 
was such that the main bulk of the trunk was made up of the 
periderm which it produced. 

Any attempt to interpret modern pteridophytes must 
clearly take into account their forerunners, now extinct, in 
the fossil record. This involves some understanding of the 
ways in which fossils came to be formed and of the extent 
to which they may be expected to provide information useful 
to the morphologist. A fossil may be defined as ^anything 
which gives evidence that an organism once hved'. Such a 
wide definition is necessary to allow the inclusion of casts, 
which are no more than impressions left in the sand by some 
organism. Yet, despite the fact that casts exhibit nothing of 
the original tissues of the organisms, they are nevertheless 
valuable in showing their shape. At the other extreme are 
petrifactions, in which the tissues are so well preserved by 
mineral substances that almost every detail of the cell walls 
is visible under the microscope. Between these two extremes 
are fossils in which decay had proceeded, to a greater or 
lesser degree, before their structure became permanent in 

the rocks. 

Under certain anaerobic conditions (e.g. in bog peat and 
marine muds), and in the absence of any petrifying mineral, 
plant tissues slowly turn to coal, in which little structure can 
be discerned, apart from the cuticles of leaves and spores. 



INTRODUCTION 23 

Portions of plants that are well separated from each other 
by sand or mud during deposition give rise to fossils known 
as mummifications or compressions. From these, it is often 
possible to make preparations of the cuticle, by oxidizing 
away the coally substance with perchloric acid. Examina- 
tion under a microscope may then reveal the outlines of the 
epidermal cells, stomata, hairs, papillae, etc. In this way, 
a great deal can be discovered from mummified leaves. 
Mummified stems and other plant organs, however, yield 
less useful results. Even their shape needs careful interpreta- 
tion, because of distortion during compression under the 
weight of overlying rocks. 

By far the most useful fossils to the palaeobotanist are 
those in which decay was prevented from starting, by the 
infiltration of some toxic substance, followed by petrifaction 
before any distortion of shape could occur. Such are, un- 
fortunately, rare indeed. The most beautiful petrifactions 
are those in silica, but carbonates of calcium and magnesium 
are also important petrifying substances. Iron pyrites, while 
common, is less satisfactory because the fine structure of the 
plant is more difiicult to observe. While it has often been 
said that during petrifaction the tissues are replaced molecule 
by molecule, this cannot be correct, for the 'cell walls' in 
such a fossil dissolve less rapidly in etching fluids than does 
the surrounding matrix. This fact forms the basis of a rapid 
technique for making thin sections of the plant material.*^ 
A poUshed surface is etched for a brief period in the appro- 
priate acid and the cell walls that remain projecting above 
the surface are then embedded in a film of cellulose acetate. 
This is stripped off and examined under the microscope 
without further treatment, the whole process having taken 
no longer than ten minutes. 

While it is frequently possible to discern the type of 
thickening on the walls of xylem elements, it is, however, 
rarely possible to make out much detail in the phloem of 
fossil plants, for this is the region which decays most rapidly. 



24 THE MORPHOLOGY OF PTERIDOPHYTES 

Furthermore, most fossils consist only of fragments of 
plants. It is then the task of the palaeobotanist to recon- 
struct, as best he can, from such partly decayed bits, the 
form, structure and mode of Ufe of the whole plant from 
which they came. There is small wonder, then, that this has 
been achieved for very few fossil plants. Many years may 
elapse before it can be said with any certainty that a particu- 
lar kind of leaf belonged to a particular kind of stem and, in 
the meantime, each must be described under a separate 
generic and specific name. In this way, the palaeobotanist 
becomes unavoidably encumbered by a multiplicity of such 
names. 

For convenience of reference, the history of the Earth is 
divided into four great eras. The first of these, the pre- 
Cambrian era, ended about 500 milUon years ago and is 
characterized by the scarcity of fossils, either of animals or of 
plants. Then came the Palaeozoic era, characterized by 
marine invertebrates, fishes and amphibians, the Mesozoic 
by reptiles and ammonites and, finally, the Cainozoic, ex- 
tending to the present day, characterized by land mammals. 
These major eras are again divided into periods (systems) 
and then subdivided again, chiefly on the basis of the fossil 
animals contained in their strata. While such a scheme is 
clearly satisfactory to the zoologist, it is less so to the 
botanist, for the plants at the beginning of one period (e.g. 
the Lower Carboniferous) are less like those of the end of 
the period (the Upper Carboniferous) than Hke those of the 
end of the previous period (the Upper Devonian). Thus, it 
is more usual for the palaeobotanist to speak of the plants of 
the Upper Devonian/Lower Carboniferous than of the 
plants of the Carboniferous period. 

The sequence of the various geological periods is summar- 
ized as a table (p. 25), in which the time scale is based on in- 
formation from R. N. C. Bowen^. Brief notes are included 
to indicate the kind of vegetation that is beheved to have 
existed during each period, but a word of caution is necessary 



GEOLOGICAL PERIODS IN IHE NORTHERN HEMISPHERE 



Era 


Period 


Age 
in 

years 


Type of vegetation 


u 


Quaternary 


I 


Modern 


o 

N 
O 

z 


Upper Tertiary, Pliocene 
Miocene 


10 

20 


Modern 


< 


Lower Tertiary, Oligocene 

Eocene 
Upper Cretaceous 


35 
50 

75 


Modern, with tropical 
plants in Europe 






u 

o 

N 


Lower Cretaceous 
Upper Jurassic 


100 

130 


Gymnosperms dominant 
(Conifers and 
Bennettitales) 


o 


Lower Jurassic (Liassic) 
Upper Triassic (Rhaetic) 


140 
160 


Luxuriant forests of 
Gymnosperms and 
Ferns 




Lower Triassic (Bunter) 
Upper Permian 


180 
190 


Sparse desert flora with 
Gymnosperms 




(Conifers and Bennetti- 
tales) 




Lower Permian 

Upper Carboniferous 
(Coal Measures) 


200 


Tall swamp forests with 
early Gymnosperms, 
Tree-Lycopods, Cala- 
mites and Ferns 


u 


Lower Carboniferous 
Upper Devonian 


250 
260 


Early Gymnosperms, 
large Tree-Lycopods 
and Ferns 


o 

N 

2 


Middle Devonian 


275 


Rhynia vegetation in 
marshy localities 


< 

Qu 


Lower Devonian 
Upper Silurian 


300 


Herbaceous marsh plants, 
{Psilophyton and 
Zosterophylhim) and 
some small shrubs 




Silurian 


350 


Marine algae 




Ordovician 


425 


Marine algae 




Cambrian 


500 


Marine algae, but some 
evidence of land plants, 
too 


PRE- 
CAMBRIAN 




4500? 


Fungi and Bacteria re- 
ported to have occur- 
red 2,000 million years 
ago 



26 THE MORPHOLOGY OF PTERIDOPHYTES 

on this matter. It must always be remembered that our 
knowledge of past vegetation is based on those fragments 
of plants that happened to become fossilized and which, 
furthermore, happen to have been unearthed. It follows, 
therefore, that a species list will certainly be biassed in favour 
of plants growing near a particular site of sedimentation and 
will not give a true picture of the world's vegetation at that 
time. Thus, until recently, it was thought that the only plants 
alive in Cambrian times were marine algae and that the land 
had not yet been colonized. This view would still be held 
today if macroscopic remains provided the only evidence, 
but recent discoveries of a wide range of cuticularized spores 
have shown that there were also numerous land plants in 
existence. Presumably, they were growing in some habitat 
where fossilization of their macroscopic remains could not 
occur. These discoveries of wind-borne spores alter the 
whole picture of Cambrian vegetation and push further back 
into antiquity the date of the first colonization of the land by 
plants. Similar considerations no doubt apply throughout 
the fossil record to a greater or lesser extent. 

We turn now to the classification of pteridophytes. The 
first object of any classification must be to group together 
similar organisms and to separate dissimilar ones. In the 
process the group is subdivided into smaller groups, each 
defined so as to encompass the organisms within it. In the 
early days of taxonomy, when few fossils were known, these 
definitions were based on living plants. Then, as more and 
more fossils were discovered, modifications became neces- 
sary in order to accommodate them, and a number of 
problems arose. The first arises from the fact that a fossil 
plant, even when properly reconstructed, is known only at 
the stage in its life-cycle at which it died. Other stages in its 
life-cycle, or in its development, may never be discovered. 
Yet, the classification of living organisms may (and indeed 
should) be based on all stages of the life-cycle. The second 
problem concerns the difficulty, when new fossils are dis- 



INTRODUCTION 



27 



covered, of deciding whether to modify the existing defini- 
tions of groups or whether to create new groups. Too many 
groups would be liable to obscure the underlying scheme of 
the classification and too few might result in each group being 
so wide in definition as to be useless. The scheme on v/hich 
this book is based is substantially the same as that proposed 
by Reimers in the 1954 edition of Engler's Syllabus der 
Pflauzenfamilien^^ and has been chosen because it seems to 
strike a balance in the number and the size of the groups 
that it contains. (An asterisk is used throughout to indicate 
fossil groups.) 



PTERIDOPHYTES 



A PSILOPHYTOPSIDA* 
Psilophy tales* 

B PSILOTOPSIDA 
Psilotales 

C LYCOPSIDA 

1 Protolepidodendrales* 

2 Lycopodiales 

3 Lepidodendrales* 

4 Isoetales 

5 Selaginellales 

D SPHENOPSIDA 

1 Hyeniales* 

2 Sphenophyllales* 

3 Calamitales* 

4 Equisetales 



E PTEROPSIDA 
a Primofilices* 

1 Cladoxylales* 

2 Coenopteridales* 

b Eusporangiatae 

1 Marattiales 

2 Ophioglossales 

c Osmundidae 
Osmundales 

d Leptosporangiatae 

1 Filicales 

2 Marsileales 

3 Salviniales 



Psilophytopsida 



Extinct plants. Only the sporophyte is known. 
Rootless, with rhizomes and aerial branches that 
are more or less dichotomous, either naked or 
with small appendages spirally arranged. Proto- 
stelic. Sporangia thick-walled, homosporous, borne 
at the tips of branches. 

Psilophytales* 
Rhyniaceae* Rhynia* Horneophyton* {=Hornea), 

Cooksonia,* Yarravia* 
Zosterophyllaceae * Zosterophyllum * 
Psilophytaceae* Psilophyton* 
Asteroxylaceae* Aster oxylon* 

The first member of this group ever to be described was 
Psilophyton princeps in 1859^^, but for many years httle 
notice was taken of this discovery. Indeed, many botanists 
regarded it almost as a figment of the imagination, so differ- 
ent was it from their preconceived ideas of land plants. 
However, by 19 17 Kidston and Lang^° had started to des- 
cribe a number of similar plants from Middle Devonian 
rocks at Rhynie in Scotland, and it became accepted that 
plants with such a simple organization had really existed. 
Only then was the group Psilophytales created to include 
them. 
The chert deposits at Rhynie, some eight feet thick, are 

28 



PSILOPHYTOPSIDA 29 

thought to represent a peat bog which became infiltrated 
with siUca. In this way the plant remains became preserved, 
some of them with great perfection. The chief plants to have 
been described from these deposits are Rhynia major, Rhynia 
Gwynne-Vaughani, Horneophyton Lignieri and Asteroxylon 
Mackiei. Of these, the first three lacked leaves as well as 
roots and are now grouped together in the Rhyniaceae along 
with Cooksonia^^ and Yarravia^^ from Upper Silurian/Lower 
Devonian rocks of Great Britain and Austraha respectively. 
The general appearance of Rhynia major is illustrated in 
Fig. 4E. It had a horizontal rhizome which branched in 
a dichotomous manner and bore groups of unicellular 
rhizoids at intervals. The tips of some rhizomes turned up- 
wards and grew into aerial stems as much as 50 cm high and 
up to 6 mm in diameter. These also branched dichotomously 
and some of them terminated in pear-shaped sporangia up 
to 12 mm long. The aerial parts were smooth and covered 
with a cuticle in which stomata were sparingly present, their 
presence indicating that the stems were green and photo- 
synthetic. In transverse section (Fig. 4F) the stems are seen 
to have had a cortex differentiated into two regions, often 
separated by a narrow zone of cells with dark contents. 
Whereas the outer cortex was of densely packed cells, the 
inner cortex had abundant inter-cellular spaces with direct 
access to the stomata; for this reason the inner cortex is 
presumed to have been the main photosynthetic region. The 
sporangium (Fig. 4H) had a massive wall, about five 
cells thick, apparently without any specialized dehiscence 
mechanism, and within it were large numbers of spores 
about 65 ju. in diameter. The fact that these spores were 
arranged in tetrads is taken to prove that they were formed 
by meiosis and that the plant bearing them represented the 
sporophyte generation. What the gametophyte might have 
looked like no one knows, though the discovery of living 
gametophytes of Psilotum containing vascular tissue has led 
to a suggestions^ that some of the bits of Rhynia, identified 



30 THE MORPHOLOGY OF PTERIDOPHYTES 

as rhizomes, could have been gametophytes. Against this 
view, however, is the fact that no archegonia or antheridia 
have yet been convincingly demonstrated. 

Rhynia Gwynne-Vaughani (Fig. 4G) was a smaller plant 
than R. major, attaining a height of only 20 cm. It was 
similar in having a creeping dichotomous rhizome with 
groups of rhizoids, but the aerial parts of the plant differed 
in several respects ; small hemispherical lumps were scattered 
over the surface and, besides branching dichotomously, the 
plant was able to branch adventitiously. An interesting 
feature of the adventitious branches was that the stele was 
not continuous with that of the main axis. It is possible that 
they were capable of growing into new plants if detached 
from the parent axis, thereby providing a means of vegeta- 
tive propagation. The sporangia were only 3 mm long and 
the spores, too, were smaller than those of i?. major. In other 
respects (internal anatomy, cuticle, stomata, etc.) the two 
species were very similar indeed. 

Horneophyton Lignieri (Fig. 4I) was smaller still, its aerial 
axes being only some 13 cm high and only 2 mm in maximum 
diameter. It was first described under the generic name 
Hornea, but in 1938 it was pointed out that this name had 
already been used for another plant and a new name was 
proposed, Horneophyton. The aerial axes were like those of 
Rhynia major, in being quite smooth and in branching 
dichotomously without any adventitious branches. In its 
underground organs, Horneophyton was very different, for 
it had short lobed tuberous corm-hke structures. From their 
upper side aerial axes grew vertically upwards and on their 
lower side were unicellular rhizoids. The stele of the aerial 
axis did not continue into the tuber, which was parenchy- 
matous throughout. Most of the tubers contained abundant 
non-septate fungal hyphae, whose mode of life has been 
the subject of some speculation. By analogy with other 
groups of pteridophytes, it is commonly supposed that there 
was a mycorrhizal association but, as Kidston and Lang^*^ 







%TTTT~TlTnf 



TTnTTTiii^ 



Fig. 4 

Zosterophyllum rhenanum: A, reconstruction; b, sporangial 
region. Yarravia oblonga: c, sporangia. Cooksonia pertoni: 
D, sporangia. Rhynia major: e, reconstruction; f, t.s. stem; 
H, sporangium. Rhynia Gwynne-Vaughani : G, reconstruction. 
Horneophyton Lignieri: i, reconstruction; J, k, sporangia 

(a, b after Krausel and Weyland; c, Lang and Cookson; d, 
Lang; e, g, h, i, j, k, Kidson and Lang) 



32 THE MORPHOLOGY OF PTERIDOPHYTES 

pointed out, some well-preserved tubers showed no trace 
whatever of fungus. This fact suggests that, instead of being 
mycorrhizal, the fungus was a saprophyte which invaded the 
tissues of the tuber after death. 

Another feature of interest, peculiar to Horneophyton, was 
the presence of a sterile columella in the sporangium (Fig. 
4J), a feature reminiscent of the mosses. One sporangium is 
illustrated (Fig. 4K) which was bifid and it is interesting 
that the columella was also bifid. This leads one to suppose 
that the stem apex could be transformed into a sporangium 
at any stage, even during the process of dichotomizing, and 
rules out any idea of the sporangium being borne by a special 
organ to which the name 'sporangiophore' might be given. 

The generic names Yarravia and Cooksonia are given to 
certain reproductive bodies detached from the plants which 
bore them ; indeed, we have no idea at all what such plants 
might have looked like. Yarravia (Fig. 4C) has been interpre- 
ted as a slender unbranched axis, terminating in a radially 
symmetrical group of five or six sporangia, partly fused 
into a synangium, about i cm long. Although Lang and 
Cookson,^^ who first described this genus, were unable to 
demonstrate the presence of spores within the sporangia this 
interpretation is widely accepted and has been used as the 
starting point for phylogenetic speculations as to the nature 
of the pollen-bearing organs of fossil seed-plants, and even 
of their seeds. Cooksonia (Fig. 4D) was much more Hke the 
other members of the Rhyniaceae, in that the sporangia 
were borne singly at the tips of tiny forking branches. Each 
sporangium was broader than it was long (one species 
being 2 mm x i mm) and contained large numbers of spores 
in tetrads. 

The chief point of difference between the Zosterophyl- 
laceae and the Rhyniaceae, described above, concerns the 
manner in which the sporangia were borne, for instead of 
terminating the main axes, they were in short terminal 
spikes, each sporangium having a short stalk. The best 



PSILOPHYTOPSIDA 33 

known genus is Zosterophyllum itself, of which three species 
have been described, one of them (Z. myretonianum) in 
considerable detail."^ ^^ Its fossil remains occur in the Old 
Red Sandstone of Scotland, and show that it grew in dense 
tufts anchored to the ground by a tangle of branching 
rhizomes. From these arose numerous erect dichotomous 
branches, 15 cm or more in height and 2 mm in diameter, 
cyhndrical and cuticularized, with a central vascular bundle 
whose xylem tracheids bore annular thickenings. An Austra- 
lian Upper Silurian species, Z. australianum, was similar,^® 
but Z. Rhenanum, described from the Lower Devonian of 
Germany,^* is said to have differed in having flattened stems. 
For this reason, it is suggested that the German species must 
have been partially submerged, as shown in the reconstruc- 
tion (Fig. 4A). The way in which the sporangia were borne 
is shown in Fig. 4B. The spikes varied in length from i cm 
to 5 cm, with the sporangia arranged spirally upon them, 
each sporangium being up to 4 mm broad. Dehiscence took 
place by means of a transverse split in the sporangium wall. 

Psilophyton, the genus which lends its name to the groups 
Psilophytopsida and Psilophytales, besides occurring in 
Canada and the United States, has also been found in 
Devonian rocks of Scandinavia, France and Belgium. 
Psilophyton princeps^^ is the best known species. Fig. 5F is a 
reconstruction of the plant. It grew to a height of about i m 
in dense clumps, arising from a tangle of creeping rhizomes 
covered with rhizoidal hairs. The aerial branches seldom 
exceeded i cm in diameter and branched profusely in a 
manner that was rather different from that of the Rhynia- 
ceae, for many of the dichotomies were unequal. In this way, 
some parts of the plant give the appearance of a sympodial 
arrangement, with a main stem and lateral branches. 

The lower parts of the aerial shoots were clothed with 
abundant outgrowths which have been variously described 
as leaves, spines and thorns (Fig. 5G). Their tips appear to 
have been glandular, they lacked stomata and vascular 

B 



34 THE MORPHOLOGY OF PTERIDOPHYTES 

supply; so none of the descriptions seems to be really 
appropriate. Since stomata were present in the cuticle 
covering the stem, it is presumed that the principal site of 
photosynthesis was in the cortex of the stem itself. However, 
only mummified specimens have been found, with the result 
that little is known about the internal anatomy of the stem, 
except that the xylem tracheids had annular or scalariform 
thickenings. 

During their growth, the aerial axes were circinately 
coiled in a manner similar to that seen in the young fronds 
of a modern fern — a method of growth which, no doubt, 
gives some protection to the dehcate stem apex, from 
mechanical damage and from desiccation. Some of the 
ultimate branches bifurcated and each fork terminated in a 
sporangium up to 6 mm long and 2 mm wide (Fig. 5H), 
within which were numerous spores in tetrads. 

Two species of Aster oxylon are known, A. Mackiei^^, 
which occurred along with Rhynia and Horneophyton in the 
Rhynie chert, and A. elberfeldense^^ from Middle Devonian 
rocks near Elberfeld, in Germany. While the German species 
is known to have attained a height of about i m, the 
Scottish species is believed to have been somewhat smaller, 
but one can only guess at its height, for only portions of the 
whole plant have been found. A. Mackiei had dichotomous 
rhizomes whose internal structure was so hke that of Rhynia 
that the two were, at first, confused. However, they were re- 
markable in being completely without rhizoidal hairs. 
Instead, small lateral branches of the rhizome grew down- 
wards into the underlying peat, branching dichotomously as 
they went, and it is assumed that they acted as the absorbing 
organs of the plant (Fig. 5A). 

The erect aerial axes were about i cm across at the base 
and they branched monopodially, dichotomies being res- 
tricted mainly to the lateral branches. Except right at the 
base, and in the presumed reproductive regions of the shoot, 
all the aerial axes were clothed with leaves arranged in a 




,i I;, y h' 



Fig. 5 
Asteroxylon Mackiei: a, reconstruction of vegetative regions; 
B, t.s. stem; c, reproductive regions; d, sporangium. Asteroxylon 
elberfeldense : e, upper portions of plant (note circinate vernation). 
Psilophyton princeps: F, reconstruction; G, enlarged portion of 
stem, showing enations; H, sporangia 

(a, c, d, after Kidson and Lang; e, Krausel and Weyland; f, 
G, H, Dawson) 



36 THE MORPHOLOGY OF PTERIDOPHYTES 

rather irregular spiral. Whatever the appendages of Psilo- 
phyton should be called, it is reasonable to call these leaves, 
for they were up to 5 mm long, were dorsiventrally flattened 
and were provided with stomata. 

Compared with Rhynia, Asteroxylon Mackiei was much 
more complex in its stem anatomy (Fig. 5B). In the centre 
was a fluted rod of tracheids which, in transverse section, 
had a stellate outhne. Some morphologists apply the term 
*actinostele' to such a structure. It was, nevertheless, a solid 
protostele, fundamentally, and its xylem consisted solely of 
tracheids, either with spiral or with annular thickenings. 
The smallest elements (protoxylem?) were near, but not 
quite at, the extremities of the ridges, with the result that the 
stele is described as mesarch. Surrounding the xylem, was a 
zone of thin-walled elongated phloem cells. The cortex was 
composed of three distinct layers, the middle one of which 
was trabecular (i.e. it consisted of a wide space, crossed by 
numerous radial plates of tissue), while the innermost and 
the outermost were of compact parenchyma. Within any 
transverse section through a leafy axis are to be seen 
numerous small vascular bundles which, although called 
*leaf traces', nevertheless stopped short without entering the 
leaves. (These are omitted from Fig. 5B, for the sake of 
clarity.) If traced inwards and downwards, they are seen to 
have had their origin in one or other of the protoxylems. 

No reproductive organs have been found actually in 
organic connection with the leafy shoots of Asteroxylon 
Mackiei, but, occurring along with them, were some 
sporangial branches which are believed to represent the 
fertile regions. These branches (Fig. 5C) were without leaves 
and terminated in small-pear shaped sporangia about i mm 
long (Fig. 5D). These contained spores in tetrads which were 
shed by means of an apical dehiscence mechanism. Whether 
these fertile branches were borne laterally or whether they 
were the apical regions of the main axis is not known. 

The appearance of Asteroxylon elberfeldense is known 



PSILOPHYTOPSIDA 37 

with more certainty and it lends support to the supposed 
reconstruction of the Scottish species. A portion of the plant 
is illustrated in Fig. 5E. The lower portions of the aerial axes 
were clothed with leaves like those of ^. Mackiei, then came 
a transition region with spine-Hke outgrowths like those of 
Psilophytoji, while the distal regions were quite smooth. 
Young developing branches were circinately coiled and the 
tips of the ultimate branchlets were frequently recurved, 
some of them bearing terminal sporangia. An interesting 
feature of its internal anatomy was the presence of a central 
pith region in the xylem of the larger axes — constituting a 
medullated protostele. 

It is impossible to overestimate the importance of the 
Psilophytopsida to botanical thought. Their discovery not 
only caused many botanists to abandon the classical theory 
that there are three fundamental categories of plant organs 
(stems, leaves and roots), but also led some of them to 
develop new and far-reaching theories of land plant evolu- 
tion. Thus, the simple Rhynia was adopted as the ideal 
starting point for the 'telome theory' of Zimmermann—, 
while Psilophyton and Asteroxylon were taken by others to 
illustrate the 'enation theory' of the evolution of leaves. 
These various theories will be discussed in greater detail in 
the final chapter; in the meantime, one should bear in mind 
the remarks of Leclercq^^ that these simple plants were by 
no means the earhest land plants, that more complex plants 
preceded them in the fossil record and that several other 
types of land plant existed alongside them in Upper Silurian/ 
Lower Devonian times. These will be described in succeed- 
ing chapters, along with the groups to which they are 
beheved to be related. 



3 

Psilotopsida 



Sporophyte rootless, with dichotomous rhizomes 
and aerial branches. Lateral appendages spirally 
arranged, scale-like or leaf-like. Protostelic (either 
solid or meduUated). Sporangia thick walled, 
homosporous, terminating very short lateral 
branches. Antherozoids flagellate. 

Psilotales 

Psilotaceae Psilotum 
Tmesipteridaceae Tmesipteris 

This small group of plants is one of great interest to mor- 
phologists because its representatives are at a stage of 
organization scarcely higher than that of some of the earUest 
land plants, despite the fact that they are living today. Their 
great simphcity has been the subject of controversy for many 
years, some morphologists interpreting it as the result of 
extensive reduction from more complex ancestors. Others 
accept it as a sign of great primitiveness. 

Two species of Psilotum are known, P. nudum {=P. 
triquetrum) and P.flaccidum {=P. complanatum), of which 
the first is widespread throughout the tropics and subtropics 
extending as far north as Florida and Hawaii and as far 
south as New Zealand. Most commonly, it is to be found 
growing erect on the ground or in crevices among rocks, but 
it may also grow as an epiphyte on tree-ferns or among 
other epiphytes on the branches of trees. P. flaccidum is a 

38 



PSILOTOPSIDA 39 

much rarer plant, occurring in Jamaica, Mexico and a few 
Pacific Islands, and is epiphytic with pendulous branches. 

The organs of attachment in both species are colourless 
rhizomes which bear numerous rhizoidal hairs and which, 
in the absence of true roots, function in their place as organs 
of absorption. In this, they are probably aided by a mycor- 
rhizal association with fungal hyphae, that gain access to the 
cortex through the rhizoids. Apical growth takes place by 
divisions of a single tetrahedral cell which is prominent 
throughout the hfe of the rhizome, except when dichotomy 
is occurring. It is said^^ that this follows upon injury to the 
apical cell as the rhizome pushes its way through the soil 
and that two new apical cells become organized in the 
adjacent regions. In any case, there is no evidence of a 
median division of the original apical cell into two equal 
halves ; to this extent, therefore, the rhizome cannot be said 
to show true dichotomy. 

In Psilotum nudum, some branches of the rhizome turn 
upwards and develop into aerial shoots, commonly about 
20 cm high, but as much as i m high in favourable habitats. 
Except right at the base, these aerial axes are green and bear 
minute appendages, usually described as 'leaves', despite the 
fact that they are without a vascular bundle (cf. Psilophyton). 
The axes branch in a regular dichotomous manner and the 
distal regions are triangular in cross-section (Fig. 6A). In 
the upper regions of the more vigorous shoots, the leaves 
are replaced by fertile appendages (Fig. 6B) whose morpho- 
logical nature has been the subject of much controversy. 
Some have regarded them as bifid sporophylls, each bearing 
a trilocular sporangium, but the interpretation favoured 
here is that they are very short lateral branches, each bearing 
two leaves and terminating in three fused sporangia. 

Psilotum flaccidum differs from P. nudum in two import- 
ant respects : its aerial branches are flattened and there are 
minute leaf-traces which, however, die out in the cortex 
without entering the leaves (cf. Asteroxylon). 



40 THE MORPHOLOGY OF PTERIDOPHYTES 

The internal anatomy of the rhizomes varies considerably, 
according to their size, for those with a diameter of less 
than I mm are composed of almost pure parenchyma, while 
large ones possess a well-developed stele. Fig. 6C is a 
diagrammatic representation of a large rhizome, as seen in 
transverse section. In the centre is a solid rod of tracheids 
with scalariform thickenings. As there is no clear distinction 
between metaxylem and protoxylem, it is impossible to 
decide whether the stele is exarch, mesarch or endarch. 
Around this is a region which is usually designated as 
phloem, although it is decidedly unlike the phloem of more 
advanced plants, for its elongated angular cells are often 
lignified in the corners. Surrounding this is a region of 
'pericycle', composed of elongated parenchymatous cells, 
and then comes an endodermis with conspicuous Casparian 
strips in the radial walls. Three zones may often be dis- 
tinguished in the cortex, the innermost of which is fre- 
quently dark brown in colour because of abundant deposits 
of phlobaphene (a substance formed from tannins by oxida- 
tion and condensation). The middle cortex consists of 
parenchymatous cells with abundant starch grains, while 
the outer cortex contains, in addition, the hyphae of the 
mycorrhizal fungus. In some cells the mycelium is actively 
growing while in others it forms amorphous partially 
digested masses. 

In the colourless, or brown, transitional region at the base 
of the aerial axes, the xylem increases in amount, becomes 
medullated and spHts up into a variable number of separate 
strands. This process of medullation continues higher up 
the stem, as shown in Fig. 6D, and the central pith region 
becomes replaced by thick-walled fibres. There is here a 
transition from the protoxylem, with its helical or annular 
helical thickenings, to scalariform metaxylem tracheids, the 
protoxylem being exarch. The xylem is surrounded by a 
region of thin-walled cells, not clearly separable as phloem 
and pericycle, and the whole stele is enclosed in a well 




Fig. 6 

Psilotimi nudum: a, portion of plant, showing erect habit; b, 
fertile region; c, t.s. rhizome; d, t.s. aerial shoot. Tmesipteris 
tannensis: E, portion of plant, showing pendulous habit; f, g, 
H, fertile appendages, viewed from different directions; i, t.s. 
aerial shoot; J, t.s. distal region of shoot; k, theoretical inter- 
pretation of sporangial apparatus of Psilotales ; l, m, abnormal 
types of sporangial apparatus 

(a, after Bold; b, e, f, g, h, Pritzel; J, Sykes) 



42 THE MORPHOLOGY OF PTERIDOPHYTES 

marked endodermis. The cortex is again divisible into three 
regions, the innermost containing phlobaphene, the middle 
region being heavily lignified and the outermost being 
photosynthetic. The chlorophyllous cells in this outermost 
region are elongated and irregularly 'sausage shaped', with 
abundant air spaces between them, which connect with the 
stomata in the epidermis. The leaves are arranged in a 
roughly spiral manner in which the angle of divergence is 
represented by the fraction i, but although internally they 
are composed of chlorophyllous cells like those of the outer 
cortex of the stem, they can contribute little to the nutrition 
of the plant, for they are without stomata as well as having 
no vascular supply. 

In this last respect, the leaves are in marked contrast to 
the fertile appendages, for these each receive a vascular 
bundle, which extends to the base of the fused sporangia, or 
even between them. In their ontogeny, too, they are markedly 
different from the leaves, for they grow by means of an 
apical cell, whereas the young leaf grows by means of 
meristematic activity at its base.^^ Shortly after the two 
leaves have been produced on its abaxial side, the apex of 
the fertile appendage ceases to grow and three sporangial 
primordia appear. Each arises as a result of perichnal 
divisions in a group of superficial cells, the outermost 
daughter cells giving rise, by further divisions, to the wall of 
the sporangium, which may be as much as five cells thick at 
maturity. The inner daughter cells provide the primary 
archesporial areas, whose further divisions result in a mass of 
small cells with dense contents. Some of these disintegrate 
to form a semi-fluid tapetum, in which are scattered groups 
of spore-mother cells, whose further division by meiosis 
gives rise to tetrads of cutinized spores. 

The genus Tmesipteris is much more restricted in its 
distribution than Psilotum, for T. tannensis is known only 
from New Zealand, Australia, Tasmania and the Polynesian 



PSILOTOPSIDA 43 

Islands, while another species, T. Vieillardi, is probably con- 
fined to New Caledonia. (Some workers recognize a further 
four species, of restricted distribution, although it is possible 
that they warrant no more than subspecific status.) T. 
tannensis most commonly grows as an epiphyte on the 
trunks of tree-ferns or, along with other epiphytes, on the 
trunks and branches of forest trees, in which case its aerial 
axes are pendulous, but occasionally it grows erect on the 
ground. By contrast, T. Vieillardi is more often terrestrial 
than epiphytic. It may further be distinguished by its 
narrower leaves and by certain details of its stelar 
anatomy. 

Like PsUotum, Tmesipteris is anchored by a dichotomous 
rhizome with rhizoidal hairs and mycorrhizal fungus hyphae. 
The aerial axes, however, seldom exceed a length of 25 cm 
and seldom branch or, if they do so, then there is but a 
single equal dichotomy. Near the base, the aerial axes bear 
minute scale-hke leaves very similar to the leaves o^ PsUotum, 
but elsewhere the branches bear much larger leaves, up to 2 cm 
long, broadly lanceolate and with a prominent mucronate 
tip (Fig. 6E). Their plane of attachment is almost unique 
in the plant kingdom, for they are bilaterally symmetrical, 
instead of being dorsiventral. They are strongly decurrent, 
with the result that the stem is angular in transverse section 
and they each receive a single vascular bundle which extends 
unbranched to the base of the mucronate tip, but does not 
enter it. In the distal regions of some shoots, the leaves are 
replaced by fertile appendages which, like those oi PsUotum, 
may be regarded as very short lateral branches, each bearing 
two leaves and terminating in fused sporangia (normally 
two) (Figs. 6F-6H). 

The internal anatomy of the rhizome is so similar to that 
of PsUotum that the same diagram (Fig. 6C) will suffice to 
represent it. In the transition region of Tmesipteris tannensis 
(Fig. 61), the central rod of tracheids becomes medullated and 
sphts up into a variable number of strands which are mesarch 



44 THE MORPHOLOGY OF PTERIDOPHYTES 

(in contrast to the exarch arrangement in P silo turn), (T. Vieil- 
lardi differs in having a strand of tracheids that continues up 
into the aerial axis in the centre of the pith region.) Whereas 
in the rhizome there is a well marked endodermis, in the 
aerial axes no such region can be discerned. Instead, be- 
tween the pericycle and the lignified cortex, all that can be 
seen is a region of cells packed with brown phlobaphene. 
The outer cortex contains chloroplasts, but the epidermis is 
heavily cutinized and is without stomata. These are res- 
tricted to the leaves (and their decurrent bases) which, like 
the stem, are also covered with a very thick cuticle, but in 
which are abundant stomata. The leaf-trace has its origin as 
a branch from one of the xylem strands in the stem and 
consists of a slender strand of protoxylem and metaxylem 
tracheids surrounded by phloem. As the stem apex is 
approached, the number of groups of xylem tracheids is 
gradually reduced (Fig. 6J), all the tracheids being scalari- 
form, even to the last single tracheid. 

The vascular strand supplying the fertile appendages 
branches into three, one to each of the abaxial leaves and 
one to the sporangial region. The latter branches into three 
again in the septum between the two sporangia. The early 
stages of development closely parallel those in Psilotum,^^ 
giving rise to thick-walled sporangia containing large num- 
bers of cutinized spores. Both sporangia dehisce simul- 
taneously, by means of a longitudinal split along the top of 
each. 

When discussing the morphological nature of the fertile 
appendages of the Psilotales, morphologists have made 
frequent reference to abnormahties^* (the study of which is 
referred to as 'teratology'). In both genera, the same types 
of variation occur, some of which are represented diagram- 
matically in Figs. 6L and 6M. The normal arrangement is 
indicated in Fig. 6K — a lateral axis (shaded) terminating in 
a sporangial region (black) and bearing two leaves (un- 
shaded). In Fig. 6L one of the leaves is replaced by a com- 



PSILOTOPSIDA 45 

plete accessory fertile appendage, while in Fig. 6M both 
leaves are so replaced and instead of the sporangial region 
there is a single leaf. There has for a long time been a widely 
held belief that freaks are 'atavistic', i.e. they are a reversion 
to an ancestral condition. However, it must be stressed that 
this beUef rests on very insecure foundations. As apphed 
here, the conclusion has been that the reproductive organs 
of the Psilotales are reduced from something more complex, 
at one time assumed to have been a fertile frond. It may well 
be, however, that the only justifiable conclusion is that, at 
this level of evolution, leaf and stem are not clearly distinct 
as morphological categories, and that they are freely inter- 
changeable — interchangeable on the fertile appendages of 
abnormal plants, just as, on any normal shoot, fertile 
appendages replace leaves in the phyllotaxy. 

Few botanists have had the good fortune to see living 
specimens of the gametophyte (prothallus) stage of either 
Psilotum or Tmesipteris, but all who have testify, not only to 
their similarity to each other, but also to their remarkable 
resemblance to portions of sporophytic rhizomes. So similar 
are the prothalU and sex organs of Psilotum to those of 
Tmesipteris that the same diagrams and descriptions will 
suffice for both. Like the rhizomes the prothalli are irregu- 
larly dichotomizing colourless cylindrical structures, covered 
with rhizoids (Fig. 7A), and the similarity is further en- 
hanced by the fact that they are also packed with mycor- 
rhizal fungus hyphae. Both archegonia and antheridia are 
borne together on the same prothallus (i.e. they are mon- 
oecious), but because of their small size they cannot be used 
in the field to distinguish prothaUi from bits of rhizomes. 
Stages in their development are illustrated in Figs. 7B — H 
(archegonia) and 7I — M (antheridia). ^^ 

The archegonium is initiated by a pericHnal division in a 
superficial cell (Figs. 7B and 7C) which cuts off an outer 
'cover cell' and an inner 'central cell'. The cover cell then 
undergoes two anticUnal divisions, followed by a series of 



46 THE MORPHOLOGY OF PTERIDOPHYTES 

periclinal divisions to give a long protruding neck, composed 
of as many as six tiers of four cells. The central cell, mean- 
time, divides to produce a 'primary ventral cell' and a 
'primary neck canal cell' (Fig. 7F). Beyond this stage 
there are several possible variants, only one of which is 
illustrated in Fig. 7G, where the primary ventral cell has 
divided to give an egg cell and a ventral canal cell, while 
the nucleus of the primary neck canal cell has divided 
without any cross wall being laid down. In the mature 
archegonium, however, most of the cells break down so as 
to provide access to the egg cell from the exterior, through a 
narrow channel between the few remaining basal cells of the 
neck, whose walls, in the meantime, have become cutinized 
(Fig. 7H). 

The antheridium, likewise, starts with a perichnal division 
in an epidermal cell (Fig. 7I). The outer cell is the 'jacket 
initial' whose further divisions in an anticlinal direction give 
rise to the single-layered antheridial wall, while the inner 
'primary spermatogenous cell' gives rise to the spermato- 
genous tissue, by means of divisions in many planes (Fig. 
7L). At maturity (Fig. 7M) the antheridium is spherical, 
projects from the surface of the prothallus and contains 
numerous spirally coiled multiflagellate antherozoids (Fig. 
7S). These escape into the surrounding film of moisture and, 
attracted presumably by some chemical substance, find their 
way by swimming to the archegonia, where fertilization 
occurs. 

Stages in the development of the young sporophyte from 
the fertihzed egg are illustrated in Figs. 7N-R. The first 
division of the zygote is in a plane at right angles to the axis 
of the archegonium (Fig. 7O) giving rise to an outer 'epibasal 
cell' and an inner 'hypobasal cell'. The latter divides re- 
peatedly to give a lobed attachment organ called a 'foot' 
(Fig. 7Q), while the epibasal cell, by repeated divisions, 
gives rise to the first rhizome, from which other rhizomes 
and aerial shoots are produced. Fig. 7R shows a young 



PSILOTOPSIDA 47 

sporophyte with three rhizomatous portions and a young 
aerial shoot, the whole plant being still attached to the 
gametophyte. This kind of embryology, where the shoot- 
forming apical cell is directed outwards through the neck of 




Fig. 7 

Psilotum nudum: a, gametophyte; b-h, stages in development of 
archegonium. Tmesipteris tannensis: i-m, stages in developing 
antheridium; n-q, stages in developing sporophyte; r, young 
sporophyte attached to prothallus ; s, spermatozoids 

(a, q, s, after Lawson; b-h, Bierhorst; i-p, r, HoUoway) 



the archegonium, is described as *exoscopic'. While relatively 
unusual in pteridophytes, it is nevertheless universal in 
mosses and hverworts. Indeed, the young sporophyte of the 
liverwort Anthoceros is very similar indeed to that of 
Tmesipteris, at least up to the stage illustrated in Fig. 7Q, 
even in such details as the lobed haustorial foot, and some 
morphologists have gone so far as to suggest some sort of 



48 THE MORPHOLOGY OF PTERIDOPHYTES 

phylogenetic relationship. However, until more is known of 
the factors which determine the polarity of developing 
embryos, such suggestions should be received with consider- 
able caution. 

For many years there has been speculation among 
botanists as to the kind of life-cycle that might have been 
exhibited by the earhest land plants. Some held the belief 
that there was a regular alternation of sporophytes and 
gametophytes that resembled each other in their vegetative 
structure and that even their reproductive organs (sporangia 
and gametangia, respectively) could be reconciled as having 
a similar basic organization: on this basis, the generations 
were regarded as 'homologous'. Others believed that the 
sporophyte generation evolved after the colonization of the 
land by gametophytic plants. From being initially very 
simple, the sporophyte then evolved into something much 
more complex, by reason of its possessing far greater poten- 
tialities than the gametophyte. On this basis, the generations 
were regarded as 'antithetic'. Until bona fide gametophytes 
are described from the Devonian, or earlier, rocks, there is 
little hope that this controversy will be resolved satis- 
factorily. All that can be done is to examine the most 
primitive living land plants and see whether, at this level 
of evolution, the sporophyte appears to have fundamentally 
different capabilities. 

The extremely close similarity in external appearance 
between the gametophytes of the Psilotales and their rhi- 
zomes is, therefore, of more than passing interest. Until 
1939, however, it was believed that there was one important 
anatomical distinction between them, in that gametophytes 
were without vascular tissue. In that year, Holloway^* des- 
cribed some abnormally large prothalli of Psilotum from 
the volcanic island of Rangitoto, in Auckland harbour. New 
Zealand. These were remarkable in having well-developed 
xylem strands, of annular and scalariform tracheids, sur- 
rounded by a region of phloem which, in turn, was enclosed 



PSILOTOPSIDA 49 

by a clearly recognizable endodermis. There was, therefore, 
almost no morphological feature distinguishing them from 
the sporophytic rhizomes, except their archegonia and 
antheridia. It was subsequently found^^ that the cells of these 
prothalli contained twice as many chromosomes as those 
from Ceylon (i.e. they were diploid), while the sporophytes 
from this locality were tetraploid. To some botanists, this 
appeared to be sufficient to explain the presence of vascular 
tissue, and tended to diminish the importance of the similar- 
ity of these gametophytes to the rhizomes. But it must be 
emphasized that diploid prothalh are known elsewhere 
among pteridophytes and that no morphological aberration 
need necessarily accompany a simple doubling of the 
chromosome number. This being so, then, whatever their 
chromosome content, these abnormal vascularized prothalli 
still provide strong support for the Homologous Theory of 
Alternation of Generations. This topic is discussed further 
in the final chapter. 

Concerning chromosome numbers generally in the group, 
it now appears that all plants of Psilotum nudum from Aus- 
traha and New Zealand have the same chromosome number 
n= 100-105, while plants from Ceylon are like Psilotum 
flaccidum in having about half this number (n=52-54). 
Tmesipteris tannensis has a chromosome number n=200+, 
while of the six new species (or subspecies) recognized by 
Barber^^ five have n=204-2io and one has n= 102-105. It is 
suggested that both Psilotum and Tmesipteris occur in poly- 
ploid series, but that both have the same basic number. 




Lycopsida 



Sporophyte with roots, stems and spirally arranged 
leaves (microphylls). Protostelic (solid or medul- 
lated) sometimes polystelic (rarely polycyclic). Some 
with secondary thickening. Sporangium thick- 
walled, homosporous or heterosporous, borne 
either on a sporophyll or associated with one. 
Antherozoids biflagellate or multiflagellate. 

1 Protolepidodendrales* 

Drepanophycaceae* Aldanophyton,"^ Baragwanathia* 

Drepanophycus * 
Protolepidodendraceae * Protolepidodendron * 

2 Lycopodiales 

Lycopodiaceae Lycopodites,^ Lycopodium, 

Phylloglossum 

3 Lepidodendrales* 

Lepidodendraceae * Lepidodendron, * Lepidophloios, * 
Bothrodendraceae* Bothrodendron"^ 
Sigillariaceae* Sigillaria* 
Pleuromeiaceae* Pleuromeia* 

4 Isoetales 

Isoetaceae Nathorstiana* Isoetes, Stylites 

5 Selaginellales 

Selaginellaceae Selaginellites* Selaginella 

50 



LYCOPSIDA 



51 



Protolepidodendrales 

Until 1953, when Aldanophyton was described from 
Cambrian deposits in Eastern Siberia, ^^ Baragwanathia^^ 
was believed to be the earhest representative of the Lycop- 
sida, for it occurs along with Yarravia in Silurian rocks of 
Australia. It had fleshy dichotomizing aerial axes, thickly 
clothed with leaves, and must have had a most remarkable 
appearance, for the diameter of the axes ranged upwards 
from I cm to 6*5 cm (Fig. 8A). In the centre was a slender 
fluted rod of annular tracheids from which leaf traces passed 




Fig. 8 

Baragwanathia : A, fertile shoot. Drepanophyciis : b, reconstruc- 
tion; c, sporophyll. Protolepidodendron: d, reconstruction; 
E, leaf scars on large axis ; F. sporophyll 

(a, after Lang and Cookson; b-f, Krausel and Weyland) 



52 THE MORPHOLOGY OF PTERIDOPHYTES 

out through the cortex into the leaves. The leaves were 
about I mm broad and up to 4 cm long and, in fertile shoots, 
they were associated with reniform sporangia arranged in 
zones. The preservation of the specimens is not good enough 
to show whether the sporangia were borne on the leaves or 
merely among them, but that they were indeed sporangia is 
estabhshed by the extraction of cutinized spores from them. 
Not much is known of the growth habit of the plant, but 
there are suggestions that the aerial branches arose from a 
creeping rhizome. Drepanophycus (=Arthrostigma) and 
Protolepidodendron both occurred in Lower and Middle 
Devonian times: the former in Germany, Canada and 
Norway; the latter in Scotland and Germany. Of the two, 
Drepanophycus (Fig. 8B) was the more robust. Its aerial 
axes were up to 5 cm thick and forked occasionally in a 
dichotomous manner. It is beheved that they arose from 
horizontal branching rhizomes. The aerial axes were covered 
with spine-like outgrowths (up to 2 cm long) in a manner 
reminiscent of Psilophyton, but with the difference that these 
outgrowths had a vascular strand and could therefore 
properly be called leaves. Some of them bore a single 
sporangium either on the adaxial surface (Fig. 8C) or in 
their axils, but these 'sporophylls' were scattered at random 
over the axes instead of being gathered together into a 
fertile zone. 

Protolepidodendron (Fig. 8D) had dichotomous creeping 
axes from which arose aerial axes up to 30 cm high and less 
than I cm in diameter. All parts of the plant were clothed 
(sometimes densely) with leaves having cushion-hke bases 
and, in most species, bifurcated apices. Stems from which 
the leaves had fallen showed a characteristic pattern of leaf- 
bases (Fig. BE) arranged in a spiral manner. All the leaves 
were provided with a single vascular strand and some of 
them bore oval sporangia on their adaxial surfaces (Fig. 8F) 
but, as in Drepanophycus, such sporophylls were not aggre- 
gated into special fertile regions. Details of the stem ana- 



LYCOPSIDA 53 

tomy of Protolepidodendron are difficult to make out, but 
there appears to have been a sohd three-angled protostele 
in the centre, with some suggestion of a mesarch proto- 
xylem. 

Whether Aldanophyton was really a member of the Lycop- 
sida cannot be determined with certainty, for no fertile 
portions of the plant have been described. It had stems up to 
13 mm in diameter, clothed with narrow leaves up to 9 mm 
long and, although the preservation of the specimens leaves 
much to be desired, one published photograph looks not 
unlike Fig. 8E. Whatever its true affinities, this plant is an 
important discovery, for it seems fairly certain that it was 
a land plant and it therefore pushes further back into 
antiquity the origin of land plants by some 200 million 
years. 



Lycopodiales 

This group contains two genera of living plants, Lycopodium 
('Club-mosses') and Phylloglossum, and one fossil genus, 
Lycopodites. Of the 200 species of Lycopodium, the majority 
are tropical in distribution, but some occur in arctic and 
alpine regions. Phylloglossum, by contrast, is monotypic and 
the single species, P. Drummondii, is restricted to New 
Zealand, Tasmania and the south-eastern corner of 
Australia. Not only do the various species of Lycopodium 
occur in widely different cHmatic regions ; they also occupy 
widely different habitats, for some are erect bog-plants, 
others are creeping or scrambling, while yet others are 
pendulous epiphytes, and this wide range of growth form is 
paralleled by an extremely wide range of anatomical 
structure. Indeed, some taxonomists have suggested that the 
genus should be split into at least four new genera, so differ- 
ent are the various species from one another. Whatever their 
status, the following sections and subsections of the genus 
are recognized by most botanists. ^^ 



54 THE MORPHOLOGY OF PTERIDOPHYTES 

A Urostachya 

1 Selago 

2 Phlegmaria 

B Rhopalostachya 

1 Inundata 

2 Clavata 

3 Cernua 

Members of the Urostachya never have creeping axes, but 
have erect or pendulous dichotomous aerial axes, according 
to whether they are terrestrial or epiphytic. Their roots 
emerge only at the base of the axes, for although they have 
their origin in more distal regions, they remain within the 
cortex (many being visible in any one transverse section of 
the stem). Perhaps the most important character, phylo- 
genetically, is the lack of specialization of the sporophylls 
which, as a result, resemble the sterile leaves more or 
less closely. Another characteristic is that vegetative repro- 
duction may frequently take place by means of bulbils. 
These are small lateral leafy stem-structures which occur in 
place of a leaf and which, on becoming detached, may 
develop into complete new plants. The members of the 
Rhopalostachya, by contrast, never reproduce by means of 
bulbils. They are all terrestrial and, although the first formed 
horizontal axes may be dichotomous, those formed later 
have the appearance of being monopodial, by reason of their 
unequal dichotomy, as also do the erect branch-systems. 
Roots may emerge from the leafy branches, particularly in 
the creeping parts of the plant. 

Of the two sections, the Urostachya (and in particular 
those belonging to the Selago subsection) are usually re- 
garded as the more primitive. The British species Lycopodium 
selago is illustrated in Fig. 9A. Its sporophylls (Fig. 9B) are 
very similar indeed to the sterile leaves (Fig. 9C) and occur 
at intervals up the stem, fertile zones alternating with sterile. 
L. squarrosum shows a sUght advance on this, in that the 




Fig. 9 

Lycopodium selago : A, plant ; b, sporophyll ; c, leaf. L. inundatum: 
D, plant; e, sporophyll; f, leaf. L. annotiniim: G, plant. L. 
clavatum: H, plant; i, sporophyll; J, leaf. L. phlegmaria: k, 
plant. L. volubile: l, plant; m, sterile branch; n, fertile branch 

(b-j, after Hooker; k-n, Pritzel) 



56 THE MORPHOLOGY OF PTERIDOPHYTES 

sporophylls are aggregated in the terminal regions of the 
axes, yet they can hardly be said to constitute a strobilus, 
for the sporophylls do not differ from the sterile leaves to 
any marked extent. All the species of the Phlegmaria sub- 
section are epiphytic. L. phlegmaria itself is illustrated in 
Fig. 9K. The pendulous dichotomous branches terminate 
in branched strobih in which the sporophylls are smaller 
and more closely packed than the sterile leaves but, 
nevertheless, afford relatively little protection for the 
sporangia. 

The Inundata subsection of the Rhopalostachya is repre- 
sented by the British species Lycopodium imindatum (Fig. 
9D). Here, the strobilus is only slightly different in appear- 
ance from the vegetative shoot, for the sporophylls (Fig. 
9E) are only sUghtly modified for protecting the sporangia 
(cf. sterile leaf. Fig. 9F). Within the Clavata subsection are 
three more British species, L. annotimim, L. clavatiim and 
L. alpinum, of which the first two are illustrated (Figs. 9G 
and 9H). In this group, the sporophylls are aggregated into 
very distinct strobili and are very different from the sterile 
leaves, for they are provided with an abaxial flange (Fig. 9I) 
which extends between and around the adjacent sporangia 
belonging to the sporophylls below (cf. sterile leaf. Fig. 9J). 
Whereas the strobih of L. annotinum terminate normal leafy 
branches, those of L. claxatum are borne on specially 
modified erect branches, whose leaves are much smaller and 
more closely appressed. There are, thus, two different kinds 
of sterile leaf in this species. The Cernua subsection includes 
a number of species with very different growth habits. 
L. cernuum has a creeping axis, from which arise at intervals 
erect branch-systems resembhng tiny fir trees in being appar- 
ently monopodial (for this reason sometimes called 'ground 
pines'). In this species, all the sterile leaves are alike, but in 
L. volubile (Fig. 9L) there are three or four kinds of sterile 
leaves. It is a plant with a scrambhng habit and its main 
axes are clothed with long needle-shaped leaves arranged 



LYCOPSIDA 57 

spirally, while the lateral branches are dorsiventral and 
superficially frond-Hke. On these branches there are four 
rows of leaves, two lateral rows of broad falcate leaves (Fig. 
9M), an upper row of medium sized needle-hke leaves and a 
row of minute hair-hke leaves along the under side. This 
species, therefore, like several others in this section is highly 
*heterophyllous'. The lateral branches in the more distal 
regions of the plant are fertile and terminate in long narrow 
strobih, which are frequently branched. As in the Clavata 
subsection, the closely appressed sporophylls have, on their 
dorsal (abaxial) side, either a bulge or a flange which pro- 
vides some protection for the sporangia below. 

The apical region of the stem in Lycopodhim differs 
markedly from species to species, for it is almost flat in 
L. selago, yet extremely convex in L. complanatum. In the 
past, opinions have diff'ered as to whether growth takes 
place from an apical cell, but it now appears that this is not 
the case^^ and that any semblance of an apical cell is an 
illusion caused by studying an apex just at the critical 
moment when one of the surface cells is undergoing an 
obhque division. All species are now held to grow by means 
of an 'apical meristem', i.e. a group of cells undergoing 
periclinal and antichnal divisions. 

The sporehngs of all species are alike in their stelar 
anatomy, for the xylem is in the form of a single rod with 
radiating flanges. In transverse section these flanges appear 
as radiating arms, commonly four in number. As the plant 
grows, the later-formed axes of most species become more 
complex, the xylem sphtting up into separate plates or into 
irregular strands. However, some species retain a simple 
stellate arrangement throughout their life, as in L. serration 
(Fig. iiF) where there are commonly five or six radiating 
arms of xylem. It is interesting that this species belongs to 
the Selago subsection which on other grounds is regarded as 
the most primitive, for some botanists, applying the doctrine 
of recapitulation, have held that the embr>'onic structure of a 



58 THE MORPHOLOGY OF PTERIDOPHYTES 

plant indicates what the ancestral adult condition was like. 
The stele of L. selago is similar to that of L. serratum, and 
the number of radiating arms of xylem may be as low as 
four. This is supposed to represent the ancestral condition 
and the Selago subsection is regarded as primitive in its 
stelar anatomy, as well as in its lack of a well defined strobilus. 
Alternating with the xylem arms are regions of phloem, 
separated from them by parenchyma, and the whole is sur- 
rounded by parenchymatous 'pericycle', outside which is an 
endodermis. The xylem strand of L. selago sometimes shows 
a slight advance on this arrangement, in that it may be 
separated into several areas with a variable number of 
radiating arms. L. clavatumhsis a number of horizontal plates 
of xylem, alternating with plates of phloem. An even greater 
number of such plates is found in L. volubile (Fig. iiC). 
To some extent this trend appears to be bound up with an 
increasing dorsiventraUty of the shoot, which reaches its 
culmination in the heterophyllous L. volubile. L. annotinum 
lends some support to this idea, for its horizontal axes are 
like those of L. clavatum, whereas its vertical axes are more 
like those of L. selago. However, exceptions are numerous 
and it may well be that no valid generalization of this kind 
can be made.*^ 

Quite a different kind of complexity is illustrated by Ly co- 
podium squarrosum (Fig. iiE), also placed in the Selago 
subsection. A transverse section of the stem of this species 
shows not only radiating arms of xylem, but also islands, 
within the xylem, lined with parenchyma and containing 
apparently isolated strands of phloem. Actually, however, 
the whole structure is an anastomosing one, so that no 
regions of phloem, or of xylem, are really isolated. This 
process of elaboration has gone even further in L. cernuum, 
where the appearance is of a sponge of xylem with phloem 
and parenchyma filHng the holes (Fig. iiD). 

Throughout the genus, the stele is exarch, the proto- 
xylem elements being clearly recognizable by their 'indirectly 



LYCOPSIDA 59 

attached annular thickenings'^^ (i.e. occasional intercon- 
nections occur between adjacent rings), while the meta- 
xylem tracheids are either scalariform or have circular 
bordered pits. The phloem consists of sieve cells which are 
elongated and pointed, with sieve areas scattered over the 
side walls. The endodermis is clearly recognizable, in young 
stems only, when casparian strips may be seen. In older axes, 
however, the walls become heavily hgnified along with the 
cells of the inner cortex and their identity becomes obscured. 
This lignified region extends through most of the cortex in 
some species, whose stems are consequently hard and wiry, 
while in other species, e.g. L. squarrosum, the stem may be 
thick and fleshy. Stomata are present in the epidermis of the 
stem and in the leaves where, in some species, they are on both 
surfaces (*amphistomatic') and, in others, only on the under 
side ('hypostomatic'). The leaves of some species are arranged 
in a whorled or a decussate manner, but in most are spirally 
arranged. However, in these, the phyllotactic fractions are 
said to be unlike those of other vascular plants in forming 
part of the series f, |, xt etc.^ (whereas the normal 
phyllotactic fractions, J, J, f, f, y\ etc., are dervied 
from the Fibonacci series). Each leaf receives a single trace, 
which has its origin in one of the protoxylems of the stem 
stele and continues into the leaf as a single unbranched vein 
composed entirely of spirally thickened tracheids. It is of 
interest that, in L. selago, the bulbils also receive this kind of 
vascular bundle, for this supports the view that, at this level 
of evolution, there is no clear morphological distinction 
between the categories 'leaf and 'stem'. This is further sup- 
ported by the fact that leaf primordia may be transformed 
by suitable surgical treatment into regenerative buds.^^ 

The so-called 'roots', too, show varying degrees of simi- 
larity to stems. All, except the first root of the sporehng, are 
adventitious and endogenous in origin, arising in the peri- 
cycle, and they are peculiar in not bearing endogenous 
laterals. Instead, they branch dichotomously (very regularly 



60 THE MORPHOLOGY OF PTERIDOPHYTES 

in some species). They are provided with a root cap and 
their root-hairs are paired (a most pecuHar arrangement). 
The majority are diarch with a crescent-shaped xylem area, 
but in some species the stele is very similar to that of the 
stem, as in Lycopodium clavatum, where the xylem takes the 
form of parallel plates. 

Variations from species to species in the shape of the 
sporophylls have already been described. In addition, there 
is considerable variation in the manner in which the 
sporangium is borne in relation to the sporophyll. In some, 
e.g. Lycopodium selago and L. inundatum, the sporangium 
is in the angle between the sporophyll and the cone axis, i.e. 
it is axillary. In others, e.g. L. cernuum and L. clavatum (Fig. 
loC), the sporangium is borne on the adaxial surface of the 
sporophyll and may be described as *epiphyllous'. The 
sporangial initials arise at a very early stage in the ontogeny 
of the strobilus, normally on the ventral side of the sporo- 
phyll, but in some species actually on the axis, whence they 
are carried by subsequent growth changes into the axil. The 
first sign of sporangial initials is the occurrence of perichnal 
divisions in a transverse row of cells (three to twelve in 
number) (Fig. loA). The innermost daughter cells provide 
the archesporial cells by further division and also contribute 
to the stalk of the sporangium, while the outermost cells (the 
jacket initials) give rise to the wall of the sporangium (Fig. 
loB). This is three cells thick just before maturity, but then 
the innermost of the layers breaks down to form a tapetal 
fluid. Like the sterile leaves, the sporophyll has a single vein, 
which passes straight out into the lamina, leaving the 
sporangium without any direct vascular supply. The mature 
sporangium is kidney-shaped and dehisces along a trans- 
verse line of thin-walled cells, so liberating the very numerous 
and minute spores into the air. 

In some species, the spores germinate without delay, while 
still on the surface of the ground, but in others there may be 
a delay of many years, by which time they may have become 




Fig. 10 

Lycopodium clavatum: A, b, c, stages in sporophyll development; 
H, prothallus; J-o, stages in archegonial development; u-z, 
embryology of young sporophyte. L. cermium: G, prothallus; 
I, archegonium; q-t, embryology. L. Selago: e, surface-livmg 
prothallus; f, subterranean prothallus; p, young sporophyte. 
Phylloglossum Drummondii: D, complete plant 
(f, foot; 1, leaf; r, root; s, suspensor; t, tuber; x, stem apex) 
(e, f, j-o, u-z, after Bruchmann; g, i, q-t, Treub) 



62 THE MORPHOLOGY OF PTERIDOPHYTES 

deeply buried. Surface living prothalli are green and photo- 
synthetic, but subterranean ones are, of necessity, colourless 
and are dependent on a mycorrhizal association for their 
successful development. Indeed, a mycorrhizal association 
appears to occur in all species growing under natural con- 
ditions, whatever their habit. As a generaUzation, it may be 
said that those species inhabiting damp tropical regions 
germinate rapidly and have green prothalli, whereas those of 
cooler regions tend to germinate slowly and produce sub- 
terranean prothalli. Lycopodium selago is interesting in this 
respect, for it shows variabihty. Fig. loE illustrates a surface 
living prothallus with photosynthetic upper regions, in 
addition to the fungal hyphae in the lower parts (and 
rhizoids). Fig. loF, on the other hand, is of a subterranean 
prothallus, with fungal hyphae in the lower regions but 
covered all over with rhizoids. Archegonia and antheridia are 
restricted to the upper parts in both cases. L. cernuum pro- 
vides an example of a surface-living prothallus (Fig. loG). 
It is roughly cyUndrical and the upper regions bear numer- 
ous green photosynthetic lobes, among which are borne the 
gametangia. In L. clavatum (Fig. loH) and L. annotinum the 
prothallus is colourless and subterranean; it is an inverted 
cone with an irregular fluted margin, growing by means of a 
marginal meristem which remains active for many years, and 
the gametangia are developed over the central part of the 
upper surface. Epiphytic species, e.g. L. phlegmaria, also 
have colourless prothalli, but they are very slender, they 
branch and they exhibit pronounced apical growth. 

Archegonia and antheridia each arise from a single super- 
ficial cell in which a periclinal division occurs. The subse- 
quent cell divisions in the antheridial initials are similar to 
those described for Tmesipteris (Fig. 7), but the mature 
antheridium differs in being sunken into the tissues of the 
prothallus. The archegonium diff'ers from that of Tmesipteris 
in having several neck canal cells, which vary in number 
according to whether the prothallus is subterranean or 



LYCOPSIDA 63 

surface living. In the latter species, the neck is very short 
e.g. Lycopodium cernuum (Fig. lol), and there may be just 
a single canal cell, apart from the ventral canal cell. At the 
other extreme, the number of canal cells may be as high as 
fourteen in L. complanatum (in the Clavata subsection), 
while L. selago is intermediate, with about seven. Various 
stages in the development of L. clavatum are illustrated in 
Figs. loJ-N. At maturity all the canal cells break down and 
part of the neck may also wither (Fig. loO). The anthero- 
zoids are pear-shaped and swim by means of two flagella 
at the anterior end, attracted chemotactically by citric acid 
diffusing from the archegonium.^* 

The orientation of the embryo in Lycopodium is endo- 
scopic and this is determined at the first division of the 
zygote, with the laying down of a cross wall in a plane at 
right angles to the axis of the archegonium (Fig. loU). The 
outermost cell, called the 'suspensor', undergoes no further 
divisions, but the innermost cell gives rise to two tiers of four 
cells, called the 'hypobasal' and 'epibasal' regions respectively 
(Fig. loW). It is from the epibasal (innermost) tier that the 
young plant is ultimately derived, by further divisions. The 
hypobasal region remains small in some species, and in 
others it swells up into a structure commonly called a *foot'. 
L. clavatum is an example of the latter and various stages are 
illustrated in Figs. loU-Z. In Fig. loX, the three regions of 
the embryo are clearly demarcated (the suspensor cell, *s' ; 
the middle hypobasal region, already beginning to swell 
into a foot, 'f; the epibasal region with a stem apex, *x', 
becoming organized), and the axis of the embryo has bent 
through a right angle. This bending of the axis proceeds 
further in Fig. loY and is completed in Fig. loZ, where, by 
turning through two right angles, the stem apex is pointing 
vertically upwards. The first root, 'r', is seen to be a lateral 
organ, not forming part of the axis of the embryo, as indeed 
is the case in all pteridophytes : not until the level of the seed 
plants does the root (radicle) form part of the embryonic 



64 THE MORPHOLOGY OF PTERIDOPHYTES 

Spindle. L. selago (Fig. loP) is similar, except that the 
hypobasal region does not swell up into a large foot. 

Lycopodium cernuum (Figs. loQ-T) is an example of a very 
different kind of embryology. As in L. selago, the hypobasal 
region remains relatively small, but the organizing of a stem 
apex is considerably delayed. The epibasal portion breaks 
through the prothalhal tissue and swells out into a tuberous 
'protocorm', 't'. Roughly spherical at the start, it is provided 
with rhizoidal hairs and mycorrhizal fungus. On its upper 
surface a cyhndrical green leaf ('protophyll'), '1', appears and 
then, as the protocorm slowly grows, further protophylls 
appear in an irregular manner. This stage may persist for a 
long time and secondary protocorms \' may be formed as 
shown in Fig. loT. Finally, however, a stem apex V 
becomes organized and a normal shoot grows out. This type 
of development has led, in the past, to much speculation as 
to its phylogenetic significance, for the protocorm was held 
by some to represent an atavistic survival of an ancestral 
condition. However, Wardlaw^^ has offered an alternative 
explanation, based on the metaboUsm of the prothallus and 
young sporophyte in the various species of Lycopodium. He 
suggests that an abnormally high carbon /nitrogen ratio may 
delay the organization of a stem apex and may lead, also, to 
a swelhng of the tissues, such being expected where mycor- 
rhizal nutrition is supplemented by photosynthesis. On this 
basis, the protocorm might well be regarded as a derivative 
and retrograde development, rather than as a sign of 
primitiveness. 

When all facts are considered, it is Lycopodium selago 
which is usually regarded as the most primitive species, in 
lacking an organized strobilus, in having a relatively simple 
vascular structure and in showing variabihty in behaviour in 
its prothallus, but such conclusions can only be speculative 
in the absence of clear fossil evidence. While there are fossil 
remains, known as Lycopodites, they contribute Uttle to 
these discussions. No petrified specimens have been found 



LYCOPSIDA 65 

and some of the mummified remains are now known to be 
those of conifers. Some had well organized strobih ; others 
did not. Lycopodites stockii, from the Lower Carboniferous 
of Scotland, appears to have been heterophyllous, with its 
leaves in whorls, and to have had a terminal cone as well as 
scattered sporophylls among the sterile leaves. Clearly, there- 
fore, this species was very different from the modern L. 
selago and, in some respects, was nearer to some members 
of the Phlegmaria subsection. 

The sporophyte of Phylloglossum Drummondii, illustrated 
in Fig. loD, is never more than about 4 cm high and appears 
above ground only during the winter months, when it 
develops a few cylindrical leaves like the protophylls of 
Lycopodium cernuum. The most robust specimens develop, 
in addition, a single erect stem terminating in a tiny strobilus. 
During the hot summer months, when the ground is baked 
hard, all the aerial parts wither and the plant survives this 
unfavourable season as a tuber. Each year a new tuber is 
formed (sometimes two or even three) from the apex of a 
lateral stem-like structure, which grows out and downwards. 
This parallel with the behaviour of the protocorm of L. 
cernuum (Fig. loT) has led to the suggestion that Phyllo- 
glossum exhibits *neoteny', in being able to produce spor- 
angia while still in an embryonic stage of development. 
Whatever the truth of this, it would certainly seem that some 
of its pecuharities are adaptations which enable it to survive 
adverse environmental conditions as a geophyte. From the 
morphogenetic point of view, it is possible to see the 
tuberization as a response to a high carbon /nitrogen ratio, 
since the prothallus is both photosynthetic and mycorrhizal. 
Perhaps all three 'causes' may apply, for they are not in- 
compatible with each other and merely represent different 
'grades of causality'. 

Chromosome counts for Phylloglossum show a haploid 
number n = about 255, with many unpaired chromosomes 
at meiosis, suggesting a high degree of hybridization in its 
c 



66 THE MORPHOLOGY OF PTERIDOPHYTES 

ancestry. Such a high number is beheved by some to be 
characteristic of primitive plants, and in this connection it 
is interesting to find that Lycopodium selago has a haploid 
number n=i30, whereas species in other sections of the 
genus have lower numbers (L. clavatum and L. annotinum 
n = 34). But such a belief is justified only as a generalization. 
High chromosome numbers may well point to ancient 
origins in the majority of cases, but not in all, for polyploidy 
could have occurred at any stage in the evolution of an 
organism. Whenever it did occur, further evolution would 
be retarded because of the masking of subsequent mutations. 
Thus, if it occurred long ago, the ancient condition would 
have become 'fixed', as may have happened in L. selago; 
whereas, if it had happened recently, it would be possible for 
an advanced morphological condition to be associated with 
a high chromosome number, as in Phylloglossum perhaps. 

By contrast with the Protolepidodendrales and the 
Lycopodiales, which are homosporous, the three remaining 
orders of the Lycopsida (Lepidodendrales, Selaginellales 
and Isoetales) are heterosporous. Another feature that they 
share is the possession of a ligule, on the basis of which they 
are sometimes grouped together as the Ligulatae. The hgule 
is a minute tongue-like membranous process, attached by a 
sunken 'glossopodium' to the adaxial surface of the leaves 
and the sporophylls. A study of hving heterosporous lyco- 
pods shows that it reaches its maximum development while 
the associated primordium of the leaf or the sporophyll is 
still quite small. The mucilaginous nature of the cells and 
the lack of a cuticle have led to the suggestion that the hgule 
may keep the growing point of young leaves and young 
sporangia moist, but the fact is that no-one knows its true 
function. It may even be a vestigial organ whose function 
has been lost. 




'\ ■S€£%?---;.'- ^ 



;60 



200 



i 




Primary Wood 



Mixed Pith 



10O 



Pith 



Protoxylem 

— • — • — .— Endodermis 



Fig. 11 

Various Lycopod steles: a, Lepidophloios Wuenschianus. b, 
Lepidodendron selaginoides. c, Lycopodium volubile. d, L. 
cernuum. E, L. squarrosum. f, L. serratum 

N.B. Leaf-traces have been omitted for the sake of clarity 

(a, b, after Hirmer; c. d, Pritzel; e, Jones) 



68 THE MORPHOLOGY OF PTERIDOPHYTES 

Lepidodendrales 

The Lepidodendrales, over 200 species of which are known, 
first appeared in Lower Carboniferous times and reached 
their greatest development in the Upper Carboniferous 
swamp forests, in which members of the Lepidodendraceae, 
Bothrodendraceae and Sigillariaceae were co-dominant with 
the Calamitales and formed forests of trees 40 m or more in 
height. The fourth family, Pleuromeiaceae, is represented 
by a much smaller plant, Pleuromeia, from Triassic rocks, 
and approached more nearly to the modern Isoetales. The 
Carboniferous genera had stout trunks, some with a crown 
of branches, others hardly branching at all, but all possessed 
the same type of underground organs, known collectively as 
Stigmarian axes. Some species of Lepidodendron, (e.g. L. 
obovatum. Fig. 12A) showed very regular dichotomies in its 
crown of branches, but others approximated to a mono- 
podial arrangement because of successive unequal dicho- 
tomies. While the trunks and branches of all species of 
Lepidodendron and Lepidophloios were protosteHc and 
exarch, there was nevertheless considerable variation in 
stelar anatomy, from species to species, and from place to 
place within one individual. Some species had soHd proto- 
steles, others meduUated protosteles; some had abundant 
secondary wood produced by a vascular cambium, some had 
little and others had none at all; in some, the stele of the 
trunk had secondary wood, while that of the branches 
lacked it altogether. Thus, Lepidodendron pettycurense and 
L. Rhodumnense (both Lower Carboniferous species) had 
soUd protosteles, the former having secondary wood in 
addition, but the latter being without it. Lepidodendron 
selaginoides ( = L, vasculare), from the Coal Measures, pro- 
vides an interesting case of partial meduUation, for the 
central region of the axis consisted of a mixture of parenchy- 
ma and tracheids, round which was a solid ring of tracheids. 
The secondary wood of this species was often excentric in 
its development, as illustrated in Fig. iiB. 



LYCOPSIDA 69 

Lepidophloios Wuenschianus, from the Lower Carbonifer- 
ous of Arran, is known in considerable detail, for examples 
have been found in which portions of the stele from various 
levels had fallen into the rotted base of the trunk before 
petrifaction occurred. This has made it possible to discover 
something about the growth processes taking place in the 
young aerial stem. The primary wood near the base was 
soUd and only 5-5 mm across, halfway up the trunk it was 
medullated, while near the top (Fig. 11 A) it was 15 mm 
across and had a hollow space in the centre of the medulla. 
It is concluded that, as the stem grew, its apical meristem 
grew more massive and laid down a much broader pro- 
cambial cylinder. Meantime, the cambium in the lower 
regions had laid down more secondary wood than higher up, 
with the result that the total diameter of the wood (primary 
and secondary together) was about the same throughout the 
length of the trunk (about 7 cm). In proportion to the over- 
all diameter of the trunk (40 cm), however, this quantity of 
wood is surprisingly small, when compared with that of a 
dicotyledonous tree, where most of the bulk is made up of 
wood. The difference probably Ues in the fact that the wood 
of modern trees is concerned with two functions, conduc- 
tion and mechanical support, whereas the wood of Lepido- 
dendrales was concerned only with conduction. Mechanical 
support was provided mainly by the thick woody periderm 
which was laid down round the periphery of the trunk. 

The metaxylem was composed of large tracheids with 
scalariform thickenings, while the protoxylem elements 
were much smaller and frequently had spiral thickenings. 
The secondary wood consisted of radial rows of scalariform 
tracheids and small wood-rays, through which leaf-traces 
passed on their way out from the protoxylem areas. In most 
specimens the phloem and even some of the cortex had 
decayed before petrifaction occurred, but what is known of 
the phloem suggests that it was small in amount and very 
similar to that of modern lycopods. 



70 THE MORPHOLOGY OF PTERIDOPHYTES 

The primary cortex was relatively thin-walled and, within 
it, a number of different regions are recognizable. Of these, 
the most interesting is the so-called 'secretory tissue', made 
up of wide thin-walled cells, whose horizontal walls became 
absorbed in the formation of longitudinal ducts. Each leaf- 
trace, as it passed through this region, acquired a strand of 
similar tissue which ran parallel with it before splitting 
into two 'parichnos strands' on entering the base of the leaf. 
It is beheved that the secretory tissue was in some way 
connected with the aeration of the underground organs, pro- 
viding an air path from the stomata of the leaves, through 
the mesophyll to the parichnos strand and so to the secretory 
zone, which was continuous with a similar region in the 
cortex of the Stigmarian axes. 

The leaves, known as Lepidophyllum, were borne in a spiral 
with an angle of divergence corresponding to some very 
high Fibonacci fraction such as -fir^, -Us, etc. They 
were Hnear, up to 20 cm long, triangular in cross-section 
and with stomata in two longitudinal grooves on the 
adaxial side. The vascular strand remained unbranched as 
it ran the length of the leaf. The leaves were shed from the 
trunk and larger branches by means of an absciss layer, and 
the shape of the remaining leaf base and scar provides im- 
portant details for distinguishing the various genera and 
species. Fig. 12B shows the appearance of the trunk of a 
Lepidodendron where, characteristically, the leaf bases were 
elongated vertically. In some species, the leaf bases became 
separated shghtly as the trunk increased in diameter, but in 
others they remained contiguous, even on the largest axes. 
No doubt this was brought about to some extent by an 
increase in the size of the leaf base, much as a leaf scar 
becomes enlarged on the bark of many angiospermous trees, 
but such increase must have been relatively slight, for other- 
wise the leaf bases would have become much broader in 
proportion to their height. Evidently, therefore, the largest 
leaf bases must have been large from the start, from which 




Fig. 12 

Lepidodendron : a, reconstruction; b, c, leaf bases. Lepidostrobus : 
D, l.s. of an idealised cone; e, t.s. sporophyll. Lepidocarpon: 
F, t.s.; G, l.s. Sigillaria: h, reconstruction; i, leaf bases. Stig- 
marian appendages: J, t.s. (1, ligule pit; 2, area of leaf base; 3, 
vascular bundle; 4, parichnos scars; 5, sporangium wall; 6, 
flange of sporophyll; 7, ligule) 

(a, b, c, h, I, after Hirmer; f, Scott; g, Hoskins and Cross) 



72 THE MORPHOLOGY OF PTERIDOPHYTES 

it follows that the axes bearing them must also have been 
large, even when young.^eb Details of Sitypical Lepidodendron 
leaf base are illustrated in Fig. 12C. Within the area of the 
leaf scar (2) are to be seen three smaller scars, representing 
the leaf-trace (3) and the two parichnos strands (4). Above 
this hes the Hgule pit (i) and, in some species, below it are 
two depressions that were once thought to be associated 
with the parichnos system, but are now known to be caused 
by shrinkage of thin-walled cells within the leaf cushion. 

Lepidophloios is distinguished by its leaf bases being 
extended horizontally, instead of vertically. Otherwise, the 
anatomy of the trunks is indistinguishable from that of 
Lepidodendron. Indeed, it has been suggested that the differ- 
ences do not warrant a separation into two genera. However, 
there were differences ; in the way the cones were borne. In 
Lepidodendron, they were nearly always terminal, whereas 
in Lepidophloios, they were borne some distance behind the 
branch tip in a cauliflorous manner. 

The cones of both genera are known as Lepidostrobus and 
they consisted of a central axis around which sporophylls 
were arranged in a compact spiral, their apices overlapping 
so as to protect the sporangia. Further protection was 
afforded by a dorsal projection, or 'heel', as illustrated in 
the ideaHzed longitudinal section. Fig. 12D. The cones 
varied in length from 5 cm to over 40 cm and must have 
looked Hke those of modern conifers. Some cones contained 
only megasporangia, others only microsporangia, while 
others were hermaphrodite. In the latter, the megasporo- 
phylls were at the base and the microsporophylls towards 
the apex, as illustrated in Fig. 12D. This is the reverse of 
the arrangement in gymnosperms and angiosperms, where 
the microsporangial organs lie below the megasporangial 
whenever they happen to be associated in a hermaphrodite 
*flower'. The sporangia of Lepidostrobus were elongated and 
attached throughout their length to the *stalk' of the sporo- 
phyll, which was relatively narrow, compared with the 



LYCOPSIDA 73 

expanded apex of the sporophyll (Fig. 12E). The sporangium 
wall was only one cell thick at maturity and dehisced along 
its upper margin. Megaspores and microspores must have 
been produced in enormous numbers, for they are extremely 
abundant in all coal-measure deposits. Some megaspores 
have been found with cellular contents, representing the 
female prothallus, retained within the megaspore wall 
('endosporic') as in Selaginella today, and occasionally 
archegonia can be recognized. 

The number of megaspores produced within each mega- 
sporangium- varied considerably from species to species, and 
in some was restricted to one. In Lepidocarpon (Figs. 12F 
and 12G) the megaspore was retained in the sporangium, 
which, in turn, was enveloped by two flanges from the stalk 
of the sporophyll. The whole structure was shed Hke a seed 
from the parent plant and has been regarded by some 
botanists as actually being a stage in the evolution of a 
seed. It would be much safer, however, to regard Lepido- 
carpon as merely analogous to a seed, for the sporophyll 
flanges are quite unlike the integuments of true seeds, except 
perhaps in function. It is not known whether the micro- 
spores germinated within the sht-hke 'micropyle' while the 
megasporophyll was still on the tree, or whether it did so 
after it had fallen to the ground. 

Sigillaria (Fig. 12H) is characterized by the arrangement 
of its leaf bases in vertical rows (Fig. 12I). It branched 
much less than Lepidodendron or Lepidophloios and it bore 
its cones in a cauliflorous manner. Furthermore, the leaves 
were much longer, up to i m, grass-Hke and, in some species, 
had two veins, possibly formed by the forking of a single 
leaf-trace. Species from the Upper Carboniferous were 
similar to Lepidodendron and Lepidophloios in their internal 
anatomy, having a medullated protostele with a continuous 
zone of primary wood. Some of the Permian species, e.g. S. 
Brardi, however, showed a further reduction of the primary 
wood, which was in the form of separate circummedullary 



74 THE MORPHOLOGY OF PTERIDOPHYTES 

Strands. This is most interesting, for it represents the 
culmination of a trend which was also taking place, at 
the same time, among several groups of early gymnosperms, 
from the sohd protostele, through medullated protosteles 
(first with mixed pith and then with pure pith) to a pith 
surrounded by separate strands of primary wood. 

From a distance, Bothrodendron must have looked very 
similar to Lepidophloios, for it had a stout trunk with a 
crown of branches covered with small lanceolate leaves and 
its cones {Bothrodendrostrobus) were borne in a cauhflorous 
manner. It differed, however, in the external appearance of 
the trunk, for it had circular leaf scars that were almost 
flush with the surface. 

The underground organs of all the genera of Lepido- 
dendrales so far described were so similar that they are all 
placed in the form genus Stigmaria, and many are placed in 
a single artificial species, S. ficoides. The base of the trunk 
bifurcated once and then immediately again, to produce 
four horizontal axes, each of which continued to branch 
dichotomously many times in a horizontal plane. These 
Stigmarian axes were most remarkable structures in many 
respects. Thus, even at their growing points, perhaps lo m 
from the parent trunk, they were frequently as thick as 
4 cm. They bore lateral appendages, commonly called 'root- 
lets', in a spiral arrangement. These were up to i cm in 
diameter and were completely without root hairs. Internally 
they show a remarkable resemblance to the rootlets of the 
modern Isoetes in having had a tiny stele separated from the 
outer cortex by a large space, except for a narrow flange of 
tissue (Fig. 12J). In origin, they were endogenous, although 
only just so. The axes on which they were borne were 
pecuhar in being completely without metaxylem. In the 
centre was either pith or a pith-cavity, round which were 
protoxylem regions directly in contact with a zone of 
secondary wood. This consisted of scalariform tracheids 
interspersed with small wood-rays, but there were also very 



LYCOPSIDA 75 

broad rays (through which the rootlet traces passed) which 
divided the wood into very characteristic wedge-shaped 
blocks. 

The true nature of Stigmarian axes has long been a 
problem to morphologists, for although doubtless they per- 
formed the functions attributed to roots in higher plants 
(absorption and anchorage), yet they were different in so 
many respects from true roots and, at the same time, were 
so different from the aerial axes that they appear to have 
belonged to a category of plant organization that was quite 
unique. Even the nature of the 'rootlets' is open to question, 
for specimens of Stigmarian axes are known which bore 
leaf-like appendages instead of rootlets. Once more one is 
forced to the conclusion that the categories root, stem and 
leaf have no clear distinction at the lower levels of evolution. 

Pleuromeia (Fig. 13 A) was a much smaller plant than the 
other members of the Lepidodendrales, for its erect un- 
branched stems were Httle more than i m high and 10 cm in 
diameter. The lower parts of the stem were covered with 
spirally arranged leaf scars, while the upper parts bore 
narrow pointed ligulate leaves about 10 cm long, attached 
by a broad base. The plant was heterosporous and dioecious, 
and the sporangia were borne in a terminal cone made up of 
numerous spirally arranged circular or reniform sporophylls. 
Although early descriptions described the sporangia as on 
the abaxial side of the sporophyll, most morphologists 
believe this to be an error and it is usually accepted that, as 
in all other lycopods, they were on the adaxial side. Verifica- 
tion of this must await the discovery of better preserved 
specimens, for no petrified material has yet been found. For 
this reason, also, httle is known of the internal anatomy of 
the plant. 

Below the ground, Pleuromeia was strikingly different 
from the other members of the Lepidodendrales, for, in- 
stead of having spreading rhizomorphs of the Stigmaria 
type, it terminated in four (or sometimes more) blunt lobes. 



76 THE MORPHOLOGY OF PTERIDOPHYTES 

From these were produced numerous slender forking root- 
lets, very similar anatomically to those of Stigmaria and also 
to those of Isoetes. Indeed, P/^wrome/a is commonly regarded 
as a Hnk connecting the Isoetales with the Carboniferous 
members of the Lepidodendrales. 



Isoetales 

Apart from the fossil genus Nathorstiana, the Isoetales con- 
tain only the two living genera Isoetes and Stylites. 

The genus Isoetes is world-wide in distribution, some 
seventy species being known, of which three occur in the 
British flora and are commonly called 'Quillworts'. /. lacus- 
tris and /. echinospora grow submerged in lakes or tarns, 
while /. hystrix favours somewhat drier habitats. Most of 
the plant is below the level of the soil, with only the distal 
parts of the sporophylls visible. These are linear structures 
from 8 to 20 cm long in /. lacustris, but up to 70 cm in some 
species growing in N. America and in Brazil. They constitute 
the only photosynthetic parts of the plant and, as in many 
aquatic plants, they contain abundant air spaces (lacunae). 
The expanded bases of the sporophylls are without chloro- 
phyll and overlap one another to form a bulb-hke structure 
which surmounts a pecuUar organ, usually referred to as a 
'corm'. The true morphology of the corm has long been the 
subject of controversy, for it is obscured by a remarkable 
process of secondary growth, involving an anomalous 
cambium. This produces small quantities of vascular tissue 
from its inner surface and large quantities of secondary 
cortex towards the outside. This secondary cortex dies each 
year, along with the sporophylls and roots attached to it, and 
it becomes sloughed off" when the new year's growth of 
secondary cortex is produced. Vertical growth of the corm 
is extremely slow, with the result that the body of the plant 
is usually wider than it is high. 

Fig. 13F is a diagrammatic representation of a vertical 



LYCOPSIDA 77 

section through an old plant of Isoetes. To the right and left 
are the shrivelled remains of the previous year's growth, the 
several sporophyll-traces and root-traces being visible 
within it. All the rest represents the present year's growth 
surrounding the perennial central regions. Occupying the 
centre is a solid protostele, the lower part of which is 
extended into two upwardly curving arms, so that the over- 
all shape resembles an anchor (Fig. 13G). This is made up of 
mixed parenchyma and pecuUar iso-diametric tracheids 
with hehcal thickenings. Towards the outside the tracheids 
are arranged in radial rows but, nevertheless, they are of 
primary origin (indeed, some workers hold that the whole 
of the primary wood is protoxylem). Surrounding this 
primary wood is a narrow zone of phloem (not shown in 
Fig. 13F), and outside this is the tissue produced centri- 
petally by the anomalous cambium. This commonly consists 
of a mixture of xylem, phloem and parenchyma and is des- 
cribed by the non-committal term 'prismatic tissue'. The 
cambium, represented in Fig. 13F as a broken line, cuts 
through the sporophyll-traces and root-traces of previous 
years, leaving their truncated stumps still in contact with 
the primary wood. 

What little vertical growth there is takes place by means 
of apical meristems at the top and bottom of the corm. The 
lower of these is extended as a line beneath the anchor- 
shaped primary xylem and is buried deeply in a groove. 
Roots arise endogenously along the sides of this groove in a 
very regular sequence and are carried round on to the under- 
sides of the newly formed cortex. The stem apex is also 
deeply sunken between the 'shoulders' of the corm and is 
said to contain a group of apical initial cells. Sporophylls 
arise in spiral sequence (with a phyllotaxy of |, t\ or 
2®i in mature plants) and, as new secondary cortex is 
formed, they are carried up on to the shoulders. 

Stages in the development of the young sporophylls are 
illustrated in Figs. 13J-L. At a very early stage, when the 




Fig. 13 

Pleuromeia: a, reconstruction. Nathorstiana : b, reconstruction. 
Stylites: c, l.s. young plant; d. l.s. older plant; e, l.s. stele. 
Isoetes: f, l.s. old plant (semi-diagrammatic); g, l.s. stele (at 



LYCOPSIDA 79 

primordium is only a few cells high, one conspicuous cell 
on its adaxial surface undergoes a periclinal division to 
produce a Hgule primordium (i). This soon gives rise to a 
membranous ligule a few mm long which, for a time, is 
much larger than the young sporophyll. Next, a velum initial 
appears, from which is developed the velum (2) — a flange of 
tissue which partly hides the sporangium in the mature 
sporophyll, except for an oval opening called the 'foramen'. 
The sporangium (3) arises as the result of pericHnal divisions 
in a group of superficial cells near the base of the sporophyll, 
on its adaxial side. The inner daughter cells are potentially 
sporogenous, while the outer (peripheral) cells give rise to 
the sporangium wall, three or four cells thick. Isoetes is 
peculiar among living plants in that some of the potentially 
sporogenous cells become organized into trabeculae of 
sterile tissue which cross the sporangium in an irregular 
manner. They subsequently become surrounded by a tapetal 
layer which is continuous with the one derived from the 
innermost layer of the sporangium wall. 

The general appearance of the base of a mature sporo- 
phyll is indicated in Figs. 13I and 13H, representing a longi- 
tudinal section and an adaxial surface view respectively. The 
sporangia of the Isoetales are larger than those of any other 
living plant and have a very high spore content indeed. The 
sporophylls formed earHest in the year and which, therefore, 
lie outermost on the apex of the corm are megasporangial 
and contain several hundred megaspores. Those formed 

right angles to f); h, leaf base (adaxial view); i, l.s. leaf base; 
J-L, development of leaf; m-o, development of male prothallus ; 
p, antherozoid ; q, r, s, development of archegonium; t-x, develop- 
ment of young sporophyte; y, megaspore with female prothallus 
and young sporophyte 

(1, ligule; 2, velum; 3, sporangium) 

(a, after Hirmer; b, Magdefrau; c, d, Rauh and Falk; f, based 
on Eames; J. k, Bower; m, n, o, Liebig; p, Dracinschi; q, r, s, 
Campbell; t-y, La Motte) 



80 THE MORPHOLOGY OF PTERIDOPHYTES 

later are microsporangial and are estimated to contain up 
to a million microspores each. Finally, a few sporophylls 
with abortive sporangia are produced late in the season. 

There is no special dehiscence mechanism and the spores 
are released only when the sporophylls die and decay, as 
they become sloughed off at the end of the season. The first 
cell division within the microspore is an unequal one which 
cuts off a small 'prothallial cell'. The other cell is called the 
'antheridial cell' since, by successive divisions (Figs. 13M 
and 13N), it gives rise to a jacket of four cells surrounding a 
central cell from which four antherozoids are formed (Fig. 
13O). These are spiral and multiflagellate (Fig. 13P) and are 
released by the cracking of the microspore wall. As already 
mentioned, this mode of development, where the prothallus 
is retained within the spore wall, is described as 'endosporic'. 

The female prothallus hkewise is endosporic. Within the 
megaspore, free nuclear divisions take place for some time, 
i.e. nuclei continue to divide without any cross-walls being 
laid down between them. Then, when about fifty such nuclei 
have become distributed round the periphery of the cyto- 
plasm, cross-walls are slowly formed, starting in the region 
immediately beneath the tri-radiate scar, but gradually 
spreading throughout. Meanwhile, the megaspore wall 
ruptures at the tri-radiate scar and an archegonium is 
formed in the cap of cellular tissue which is thereby exposed. 
Stages in the development of the archegonium are illustrated 
in Figs. 13Q-S. If fertiUzation does not occur immediately, 
further archegonia may develop among the rhizoids that 
cover the apex of the gametophyte. 

Stages in the development of the young sporophyte are 
illustrated in Figs. 13 T-Y, in which the megaspore is sup- 
posed to be lying on its side, as is commonly the case. The 
first division of the zygote is in a plane at right angles to the 
axis of the archegonium, or sUghtly oblique to it. That part 
of the embryo formed from the outermost half, designated 
'the foot', is indicated in the figures by obhque shading. As 



LYCOPSIDA 8l 

growth proceeds, the orientation of the embryo changes so 
that the first leaf and the stem apex are directed upwards, 
while the first root is directed obliquely downwards. It is of 
interest that there is no quadrant specifically destined to 
produce a stem apex, and that it appears relatively late in a 
position somewhere between the first leaf and the first root. 
In some species, there are no clearly defined quadrants at all. 

Despite the absence of a suspensor, the embryology of 
Isoetes may be described as endoscopic, since it is from the 
inner half of the dividing zygote that the shoot is ultimately 
formed. 

For some time, the young embryo continues to be en- 
closed within a sheath of prothaUial tissue which grows out 
round it, but ultimately the various organs break through 
and the first root penetrates the soil. 

A chromosome count of n= lo has been obtained in one 
species of Isoetes, and of n = 54-56 in several others. 

Isoetes is clearly a remarkable genus, not only in its 
pecuhar method of secondary thickening, but also in the 
fact that all its leaves are, at least potentially, sporophylls. 
For this reason, some morphologists regard the upper half 
of the corm as representing a cone axis. The lower half they 
regard as a highly reduced rhizomorph, homologous with 
Stigmarian axes, and this is supported, not only by the 
regular arrangement of the roots on the corm, but also by 
the extraordinary similarity of the roots to Stigmarian root- 
lets internally. If this view is correct, then the stem, as such, 
must have become completely suppressed, along with its 
leaves. 

Stylites was unknown until 1954, when it was first dis- 
covered, forming large tussocks round the margins of a lake 
at an altitude of 4,750 m in the Peruvian Andes. Since then, 
it has been examined in great detail by Rauh and Falk^^, 
who claim that there are two species. Stylites is no less re- 
markable than Isoetes, for it likewise exhibits a kind of 
anomalous secondary thickening, though less active, and all 



82 THE MORPHOLOGY OF PTERIDOPHYTES 

its leaves are potential sporophylls. It differs from Isoetes, 
however, in having hmited powers of vertical growth and in 
being able to branch, both dichotomously and adventitiously, 
so as to form the characteristic tussocks. Two plants are 
illustrated in longitudinal section, one young and un- 
branched (Fig. 13C), the other older and branching (Fig. 
1 3D). Perhaps the most remarkable feature is the way in 
which the roots are borne up one side only and receive their 
vascular supply from a rod of primary wood which is quite 
distinct from that supplying the sporophylls; the two run 
side by side within the axis (Fig. 13E). The nature of the 
axis is, therefore, even harder to interpret than in Isoetes. 
Rauh and Falk draw a comparison with the Cretaceous 
Nathorstiana (Fig. 13B) in which the roots arise from a 
number of vertical ridges round the base of the stem. This 
in turn may be compared with Pleuromeia and ultimately, 
therefore, with the Lepidodendrales. 

Selaginellales 

This group contains two genera, one living (Selaginella) and 
one fossiUzed {Selaginellites). More than 700 species of 
Selaginella are known, of which some occur in temperate 
regions, but the vast majority are confined to the tropics and 
subtropics, where they grow in humid and poorly illuminated 
habitats, such as the floor of rain-forests. Some, however, 
are markedly xerophytic, inhabiting desert regions, and are 
sometimes called 'resurrection plants' because of their 
extraordinary powers of recovery after prolonged drought. 
Relatively few are epiphytes, unlike Lycopodium. Some 
form delicate green mossy cushions, others are vine-Uke, 
with stems growing to a height of several metres, while 
many have creeping axes, from which arise leafy branch 
systems that bear a striking superficial resemblance to a 
frond. 

Hieronymus^2 divided the genus into the following 

sections and subsections : 



LYCOPSIDA 83 

A Homoeophyllum 

1 Cylindrostachya 

2 Tetragonostachya 

B Heterophyllum 

1 Pleiomacrosporangiatae 

2 Oligomacrosporangiatae 

The Homoeophyllum section is a small one, consisting of 
fewer than fifty species, all of which are isophyllous and have 
spirally arranged leaves. The only native British species, 
Selaginella selaginoides { = S. spinosa, = S. spinulosa) (Fig. 
14A), is a typical example of this kind of organization and is 
placed in the subsection CyUndrostachya because the spiral 
arrangement extends also to the fertile regions. Species 
belonging to the Tetragonostachya subsection differ in that 
the sporophylls are arranged in four vertical rows, giving the 
cone a four-angled appearance. All the members of the 
Homoeophyllum section are monostelic, but S. selaginoides is 
pecuhar in that the stele of the creeping region is endarch 
(Fig. 14I), whereas that of the later-formed axes is exarch 
(Fig. 14H), as in all other species. According to Bruchmann^ 
there is a Umited amount of secondary thickening in the so- 
called hypocotyl region of this species; this is the only 
record of cambial activity in the whole genus. 

The Heterophyllum section is characterized by a markedly 
dorsiventral symmetry and by anisophylly, for the leaves are 
arranged in four rows along the axis, two rows of small leaves 
attached to the upper side and two of larger ones attached 
laterally. The fertile regions, however, are isophyllous and 
the cones are four-angled, which makes them very clearly 
distinguishable from the vegetative regions (Fig. 14F). The 
section is divided, somewhat arbitrarily, on the number of 
megasporophylls in the cone and is further subdivided on the 
number of steles in the axis. 

Most commonly the axis is monostelic and contains a 
ribbon-shaped stele, e.g. Selaginella flabellata (Fig. 14K), but 




> / 



* .^'.'■■'^IVS 



K 



I' liif iTfT til lff' fif^' 



1 ; ^ 



L (^J 




Fig. 14 

Selaginella selaginoides : a, whole plant; b, microsporophyll; 
c, megasporophyll. S. Braunii: e, portion of plant; f, branch 
tip + cone. S. Willdenowii : G, attachment of rhizophore. 



LYCOPSIDA 85 

some species have more complicated stelar arrangements. 
S. Kraussiana, now naturalized in parts of the British Isles, 
has a creeping habit and has two steles which run side by 
side (Fig. 14 J), except at the nodes where they interconnect. 
S. Braunii (Fig. 14E) is one of the many species which have a 
creeping stem with erect frond-like branch systems: the 
creeping axis is bi-stelic, with one stele lying vertically above 
the other, while the erect axes are monostelic. S. Willdenowii 
is a climbing, or vine-like, species and may have three 
ribbon shaped steles (Fig. 14L) or even four. The most 
complex of all is S. Lyallii, where the creeping axis is di- 
cycHc and the aerial axes are polystelic. The central stele of 
the creeping axis is a simple ribbon of metaxylem, without 
any protoxylem, surrounded by phloem, pericycle and endo- 
dermis. This is surrounded by a cylindrical stele which is 
amphiphloic (i.e. has phloem to the inside as well as to the 
outside of the xylem) and is bounded, both externally and 
internally, by endodermis. Both steles play a part in the 
origin of the many steles in the aerial axis, which number 
twelve or thirteen, four of them being main ones to which the 
leaf traces are connected, while the rest are accessory steles. 
It is important to realize that, however complex the stem 
of a mature plant may be, the young sporeUng Selaginella is 
invariably monostehc, there being a gradual transition along 
the axis until the adult condition is achieved. This observa- 
tion has naturally, in the past, led to the supposition that 



Steles : S. selaginoides, h (aerial axis), i (creeping axis) ; 
S. Kraussiana, j; S. flabellata, k; S. Willdenowii, L. 
Embryology: S. Martensii, m-p; S. selaginoides, q; 
S. Poulteri, R, s, t; S. Kraussiana, u, v; 
S. Galleottii, w ; S. denticulata, x. 
Biflagellate sperm, y 

(1, ligule; 2, rhizophore; 3, diaphragm; a, archegonial tube; 
f, foot; r, root; s, suspensor; x, stem apex) 

(a, b, c, f, after Hieronymus; h-l, Gibson; m-x, Bruchmann; v, 
Dracinschi) 



86 THE MORPHOLOGY OF PTERIDOPHYTES 

Species which are monosteUc throughout are more primitive 
than the more complex species. However, much caution is 
necessary before accepting this view. In the first place, not 
all monosteles are directly comparable (e.g. S. selaginoides 
and S. Braunii). In the second place, it would seem that 
some members of the monostelic Homoeophyllum group 
are highly advanced in other respects. Thus, S. rupestris and 
S. oregana (in the Tetragonostachya subsection) are re- 
markable for the possession of vessels in their xylem. 
Among the tracheids are Hgnified elements whose transverse 
end-walls have dissolved, leaving a single large perforation 
plate so that, hke drain pipes placed end to end, they provide 
long continuous tubes. While it is true that vessels are known 
in some other pteridophytes, they are not of this advanced 
type which occurs, elsewhere, only among the flowering 
plants. The metaxylem tracheids have scalariform thicken- 
ings, while the protoxylem elements are helically thickened 
and exhibit a feature which is found elsewhere only in 
Isoetes — viz. the helix may be wound in different directions 
in different parts of the cell.^^ 

The phloem, composed of parenchyma and sieve cells, is 
very similar to that of Lycopodium and is separated from 
the xylem by a region of parenchyma one or two cells 
thick. To the outside of it is a region of pericycle, and then 
comes a trabecular zone which is characteristic of Selaginella. 
This zone differs markedly in detail from species to species, 
but is usually a space, crossed in an irregular fashion by 
tubular cells or by chains of parenchyma cells. Endodermal 
cells are recognizable also in this region because of their 
Casparian bands, but it sometimes happens that a single 
Casparian band may encircle a bunch of several tubular 
cells. Whatever the exact constitution of the zone, however, 
it is very deUcate, with the result that the stele usually drops 
out of sections cut by hand. The outer regions of the stem 
are frequently made up of thick-walled cells and the epi- 
dermis is said to be completely without stomata. 



LYCOPSIDA 87 

In Selaginella selaginoides, roots arise in regular sequence 
from a swollen knot of tissue in the hypocotyl region, but 
in most creeping species they arise at intervals along the 
under side of the stem. They are simple monarch structures, 
which branch dichotomously in planes successively at right 
angles to each other, as they grow downwards into the soil. 
Root-caps and root-hairs are present, just as in the roots of 
other plants. A mycorrhizal association has been demon- 
strated in S. selaginoides. In species with aerial branches, the 
roots are associated with pecuhar organs, usually referred 
to as *rhizophores', and some morphologists describe the 
roots as borne on them, while others describe the rhizo- 
phores as changing into roots on reaching the soil. Of these, 
the second interpretation is probably the more accurate. 
Rhizophores are particularly well developed in cHmbing 
species, such as S. Willdenowii (Fig. 14G), where they grow 
out from 'angle meristems' which occur in pairs, one above 
and one below, at the junction of two branches. In some 
species, only one of these is active while the other remains 
dormant, as a small papilla. The active one grows into a 
smooth shiny forking structure without leaves. Its branches 
are without root-caps until they reach the soil, but then root- 
caps appear and all subsequent branches take on the appear- 
ance of typical roots. This is the normal behaviour, but the 
fate of the angle meristems appears to be under the influence 
of auxin concentrations, for damage to the adjacent branches 
may result in their giving rise to leafy shoots, instead of 
rhizophores. It is clear that the rhizophore fits neither into 
the category 'stem' nor into the category 'root', but exhibits 
some of the characters of each. It is not surprising, therefore, 
that in the days when botanists believed in the reality of 
these morphological categories, the rhizophores of Selagin- 
ella were the subject of much argument. 

The stem apex shows an interesting range of organiza- 
tion, from species to species, for those with spirally arranged 
leaves tend to have a group of initial cells, while dorsiventral 



88 THE MORPHOLOGY OF PTERIDOPHYTES 

Species usually have a single tetrahedral apical cell.^* Leaf 
primordia are formed very close to the stem apex and, in 
some species, appear to arise from a single cell. They give 
rise to typical microphylls, receiving a single vascular bundle 
which continues into the lamina as an unbranched vein. The 
ligule, which is present on every leaf and sporophyll, appears 
early in their ontogeny and develops from a row of cells 
arranged transversely across the adaxial surface near the base 
of the primordium. When fully grown it may be fan-shaped 
or lanceolate and has a swollen *glossopodium' sunken into 
the tissue of the leaf. There is much variation, according to 
species, in the structure of the lamina of the leaf, for some 
species possess only spongy mesophyll, while others have a 
clearly defined palisade layer also. In some, the cells of the 
upper epidermis and, in others, some of the mesophyll cells 
contain only a single large chloroplast, a feature which is 
reminiscent of the liverwort Anthoceros. In other species, all 
the cells of the leaf contain several chloroplasts. There is 
much variation, also, in the occurrence of stomata, some 
species being amphistomatic and others hypostomatic. 

Early stages in the development of the sporangia in 
Selaginella are very similar to those in Lycopodium, and there 
is a similar range of variation in the location of the primor- 
dium. Thus, in some species, it arises on the axis, while in 
others it arises on the adaxial surface of the leaf, between the 
ligule and the axis. However, at maturity the sporangium 
comes to lie in the axil of the sporophyll. The first division 
is periclinal and gives rise to outer jacket initials and inner 
archesporial cells. The jacket initials divide further to pro- 
duce a two-layered sporangium wall and the archesporial 
cells produce a mass of potentially sporogenous tissue, sur- 
rounded by a tapetum. In microsporangia many cells of the 
sporogenous tissue undergo meiosis to form tetrads of 
microspores but, in the megasporangia of most species, all 
the sporogenous tissue disintegrates, except for one spore 
mother cell, from which four megaspores are formed. Some 



LYCOPSIDA 89 

Species, however, retain more than the one functional mega- 
spore mother cell, so that up to twelve or even more mega- 
spores may result. Yet other species are pecuhar in that, out 
of the single tetrad of megaspores, one, two or three may be 
abortive, so that in the extreme condition the megasporan- 
gium may contain only one functional megaspore. S. rupestris 
usually has two megaspores in each sporangium and some- 
times only one, while S. sulcata regularly has only one. 
S. rupestris is further remarkable in that the megaspores are 
not shed, but are retained within the dehisced megasporan- 
gium and fertilization takes place while it is still in situ. 
Thus, it happens that young sporophytes may be seen grow- 
ing from the cone of the parent sporophyte. Few seed-plants 
have achieved this degree of vivipary, yet in Selaginella it 
occurs in a species belonging to the allegedly primitive 
Homoeophyllum group. 

In those species whose cones contain both megasporo- 
phylls and microsporophylls, it is usual for the former to be 
near the base of the cone and the latter near the apex. This 
further emphasizes the point, already made, that the arrange- 
ment in lycopods is the inverse of that observed both in the 
gymnospermous Bennettitales and in hermaphrodite flowers 
of angiosperms. 

Fig. 14D illustrates the appearance of a megasporophyll 
in longitudinal section, with the hgule (i) and the differential 
thickening in the sporangium wall, while Figs. 14B and 14C 
illustrate a dehiscing microsporangium and megasporangium 
respectively. Contractions of the thick-walled cells of the 
megasporangium cause the megaspores to be ejected for a 
distance of several centimetres, but dispersal of the micro- 
spores is mainly by wind currents. 

Long before the spores are shed, nuclear divisions have 
started to take place, so that the prothallus is well advanced 
when dehiscence occurs. The stages in the formation of the 
male prothallus are very similar to those figured for Isoetes 
and, at the moment of Uberation, the male prothallus 



90 THE MORPHOLOGY OF PTERIDOPHYTES 

commonly consists of thirteen cells (one small prothallial 
cell, eight jacket cells and four primary spermatogenous 
cells, of which the latter undergo further divisions to produce 
128 or 256 biflagellate antherozoids— Fig. 14Y). Within the 
megaspore, a large vacuole appears, around which free 
nuclear divisions occur and then, subsequently, a cap of 
cellular tissue becomes organized beneath the tri-radiate 
scar. In some species, this cap is continuous with the rest of 
the prothallus, which later becomes cellular too, but in 
others a diaphragm of thickened cell walls is laid down, as 
illustrated in Figs. 14R and 14U (3). Rupturing of the mega- 
spore allows the cap to become exposed and it frequently 
develops prominent lobes of tissue, covered with rhizoids, 
between which are numerous archegonia. It has been 
suggested that the rhizoids, as well as anchoring the mega- 
spore, may serve to entangle microspores in close proximity 
to the archegonia. 

The archegonia are similar to those of Isoetes, except that 
the neck is shorter and consists of two tiers of cells only. 

The embryology of Selaginella is remarkable for the very 
great differences that occur between species. These are 
illustrated in Figs. 14M-X, all of which, for ease of com- 
parison, are drawn as if the megaspore were lying on its 
side. The first cross wall is in a plane at right angles to the 
axis of the archegonium (Fig. 14M) and the fate of the 
outermost half in the different species is indicated by 
obUque shading. In S. Martensii (Figs. 14M-P) the outer half 
gives rise to a suspensor (s), while the inner half gives rise to 
all the rest of the embryo, with a shoot apex, (x), a root (r) 
and a swollen foot (f). The axis of the embryo, in this species, 
becomes bent through one right angle so as to bring the shoot 
apex into a vertical position. S. selaginoides (Fig. 14Q) 
is similar, except for the absence of a foot. S. Poulteri (Figs. 
14R-T) is a species with a well developed diaphragm, through 
which the embryo is pushed by the elongating suspensor. A 
curvature through three right angles then brings the shoot into 



LYCOPSIDA 91 

a vertical position. S. Kraussiana (Figs. 14U and 14V) likewise 
has a diaphragm, but in this species the venter of the arche- 
gonium gradually extends through it (a), so carrying the 
embryo with it into the centre of the prothallus. The outerhalf 
of the dividing zygote provides, not only the vestigial suspensor, 
but also the foot. The archegonium of S. Galeottii behaves in a 
similar way, but the embryo is different (Fig. 14W) in that 
the outer half provides the suspensor, the foot and also the 
root. S. denticulata (Fig. 14X) has the various parts of the 
embryo disposed as in S. Martensii (i.e. the root lies between 
the suspensor and the foot) but they are derived in a com- 
pletely different way, for they all come from the outermost 
half of the dividing zygote. 

Such extraordinary variations as these are very puzzHng 
and have occupied the thoughts of many morphologists. 
Some have held that the presence of a well developed sus- 
pensor is a primitive character and that the reduction of this 
organ in some species is a sign of relative advancement. 
Its reduction seems to be correlated with the transference 
of its function to the venter of the archegonium, and 
this would certainly seem to be an advanced condition. As 
to the *foot', all that can be said is that it has Httle reality 
as a separate organ, since it can apparently be formed from 
various regions of the embryo and may even be dispensed 
with altogether. 

Selaginella is pecuUar among pteridophytes for its low 
chromosome numbers, n = 9 being the commonest number, 
and its chrom.osomes are minute. 

Selaginellites is the genus to which are assigned all fossil 
remains of herbaceous lycopods that are known to have been 
heterosporous. The recent examination of Selaginellites crassi- 
cinctus'^^ is of particular interest, for within its cones were 
found the megaspores Triletes triangularis, which have long 
been known as one of the commonest spores in coal measure 
deposits, but whose origin was hitherto unknown. This dis- 
covery suggests that Selaginellites v^sls probably an important 



92 THE MORPHOLOGY OF PTERIDOPHYTES 

component of the flora of those times, contemporaneous 
with the tree-Hke Lepidodendrales. Whereas this species was 
similar to most Selaginella species in having four megaspores 
in each sporangium, others had sixteen or even thirty-two, 
which suggests that they had not progressed so far in the 
direction of heterospory. While there is no general agree- 
ment among botanists as to how the various groups of the 
Lycopsida are related to each other, it is generally supposed 
that the heterosporous forms must have evolved from some 
homosporous ancestor. 

In this connection, it is perhaps significant that, among 
Selaginella species, the type regarded as the most primitive 
(S. selaginoides) approaches most nearly to the Lycopodium 
species which is regarded as the most primitive (L. selago). 
Both are erect and isophyllous, with spirally arranged leaves 
showing the least difference between fertile and sterile 
regions and both having simple protostehc vascular systems. 
The similarities extend even to the young embryo, as a com- 
parison of Figs. loP and 14Q will show. The lack of a well 
developed foot in each is interesting, and makes one wonder 
whether it might have been absent from their ancestors also. 

The most important differences between these two plants, 
therefore, seem to be the heterospory of Selaginella and its 
possession of a hgule. If it be accepted that heterospory is 
derived from homospory, there remains only the ligule to be 
explained. This is, indeed, difficult. There is no obvious 
reason why, in lycopods, this structure should invariably be 
associated with heterospory. Selaginella is usually grouped 
with Isoetes and the Lepidodendrales on the basis of the 
possession of these two characters, yet on other grounds 
Selaginella stands apart from Isoetes. The multiflagellate 
antherozoids of the latter suggest very fundamental differ- 
ences. On the basis of the number of flagella, Lycopodium 
and Selaginella should be grouped together. Unfortunately, 
of course, we have no knowledge of the antherozoids of the 
Lepidodendrales, but one's guess would be that they were 



LYCOPSIDA 93 

multiflagellate, like those of Isoetes and Stylites. One thing 
is fairly certain — that Selaginella is not a direct descendant 
of the Lepidodendrales. Apart from this, one's views on the 
relationships of the Lycopsida must depend upon a decision 
as to whether the ligule is more significant phylogenetically 
than the number of flagella. 



Sphenopsida 



Sporophyte with roots, stems and whorled leaves. 
Protostelic (solid or medullated). Some with secon- 
dary thickening. Sporangia thick-walled, homo- 
sporous (or heterosporous), usually borne in a re- 
flexed position on sporangiophores arranged in 
whorls. Antherozoids multiflagellate. 



Hyeniales* 

Protohyeniaceae* Protohyenia* 
Hyeniaceae* Hyenia* Calamophyton 



* 



2 Sphenophyllales* 

Sphenophyllaceae* Sphenophyllum* Sphenophyllos- 

tachys* Bowmanites,* Eviostachya* 
Cheirostrobaceae* Cheirostrobus* 

3 Calamitales* 

Asterocalamitaceae* Asterocalamites* Archaeocala- 

mites* 

Calamitaceae* Protocalamites* Calamites,* 

Annularia,'^ Asterophyllites,* 
Pro tocalamostachys, * Calamo- 
stachys* Palaeostachya* 

4 Equisetales 

Equisetaceae Equisetites,* Equisetum 

94 



SPHENOPSIDA 95 

Hyeniales 

Until 1957 the earliest known representatives of the Sphen- 
opsida were Hyenia and Calamophyton, both of which are of 
Middle Devonian age, but in that year Ananiev discovered 
the remains of a most interesting plant in Lower Devonian 
rocks of western Siberia. This he named Protohyenia (Fig. 
1 5 A). Although lacking some of the features which are 
characteristic of the Sphenopsida, yet, as the generic name 
suggests, it might well represent an early ancestor of the 
group. From a creeping axis, erect branches arose at inter- 
vals, bearing either sterile or fertile appendages in rather 
indefinite whorls. The sterile appendages forked several 
times and, although having the appearance of tiny lateral 
branches, they probably functioned as leaves. The fertile 
appendages were very similar, but terminated in sporangia. 
These were unUke those of almost all other members of the 
Sphenopsida, in that they were not reflexed. 

Hyenia elegans (Fig. 15E) had a similar growth habit, as 
we now reahze from the work of Leclercq,^^ for it had a stout 
horizontal rhizome bearing roots and erect aerial stems up to 
30 cm high, some sterile and others fertile. The sterile axes 
bore whorls of forking appendages, alternating at successive 
nodes and, as in the case of Protohyenia, it is difficult to 
decide whether they should be regarded as leaves or as stems 
performing the functions of leaves. The fertile axes bore 
whorls of sporangiophores (Fig. 15F), which were similar to 
the 'leaves', except that two segments were reflexed and 
usually terminated in two sporangia each. 

Other species of Hyenia are known, in which the aerial 
axes were branched and which, therefore, resembled quite 
closely the other Middle Devonian genus Calamophyton. 
The illustration of C. primaevum (Fig. 15B) is taken from an 
early description^^ which emphasizes the articulate nature of 
the aerial axes, a feature which used to be regarded as 
essential in defining the genus. However, other species, e.g. 
C. bicephalum, are not so clearly articulated and it has been 




Fig. 15 

Protohyeniajanovii: A, reconstruction. Calamophyton: b, recon- 
struction of C. primaevum; c, leaf, and d, sporangiophore of 
C. bicephalum. Hyenia elegans: E, reconstruction; f, sporan- 
giophore. Eviostachya Hoegi: G, sporangiophore; h, mode of 



SPHENOPSIDA 97 

suggested that the two genera merge into one another.®^ 
Certainly the sterile and fertile appendages of C. bicephalum 
(Figs. 15C and 15D) were very similar to those of Hyenia, 
the chief difference being that the fertile appendages of the 
former forked more profusely and bore twelve sporangia, 
instead of three or four, as in the latter. The lateral appen- 
dages of C. primaevum are said to have been much simpler, 
forking only once, and the sporangiophores are said to have 
borne only two sporangia; the way in which they were 
restricted to special fertile branches may represent early 
stages in the evolution of the strobilus, which is so character- 
istic of later sphenopsids. 

Little is known of the internal anatomy of the Hyeniales, 
but in Calamophyton there are indications of a triangle of 
pith surrounded by tracheids with reticulate or scalariform 
thickenings; it is also suggested that there may have been 
some degree of secondary thickening. 

Sphenophyllales 

This group first appeared in the Upper Devonian and per- 
sisted until the Lower Triassic, remains of stems as well as of 
leaves being referred to the genus Sphenophyllum. Many 
species are known, all of which are characterized by the 
whorled arrangement of the leaves, usually in multiples of 
three at each node (Fig. 15K). The stems were usually very 
delicate, in spite of secondary thickening, for they seldom 
exceeded i cm in diameter. Presumably, therefore, they 

branching of sporangiophore. Cheirostrobus pettycurensis : i, 
sporangiophore and bract. Sphenophyllostachys (= Bowmanites) 
fertilis: J, reconstruction of part of cone. Sphenophyllum cunei- 
folium: k, reconstruction; l, stele; m-o, leaves. Sphenophyllo- 
stachys Dawsoni: p, part of cone in l.s.; Q, part of cone in t.s. 
Sphenophyllostachys Roemeri: r, part of cone in t.s. 

(a, after Ananiev; b, Krausel and Weyland; c, d, Leclercq and 
Andrews; e-h, j, Leclercq; i, Scott; k, Smith; m-o, Jongmans; 
p-R, Hirmer) 
D 



98 THE MORPHOLOGY OF PTERIDOPHYTES 

were unable to support their own weight and must have been 
prostrate on the ground, or must have depended on other 
plants for support. In general appearance, they probably 
looked rather hke a Galium ('Bedstraw'). The anatomy of 
the stem was most peculiar in its resemblance to that of a 
root, for in the centre was a triangular region of solid prim- 
ary wood, with the protoxylems at the three corners in an 
exarch position. In the Lower Carboniferous species, S, 
insigne, the protoxylem tended to break down to form a 
'carinal' canal, but in the Upper Carboniferous species this 
rarely happened. Outside the primary wood, a vascular 
cambium gave rise to secondary wood, first between the 
protoxylems, and then later extending all round. However, 
the wood opposite the protoxylems was composed of smaller 
cells than on the intermediate radii, resulting in a pattern 
which is quite characteristic and which is recognizable at a 
glance in transverse sections (Fig. 15L). The primary wood 
consisted entirely of tracheids (i.e. without any admixture of 
parenchyma) and they bore multiseriate bordered pits on 
their lateral walls. The tracheids of the secondary wood also 
bore multiseriate pits, but they were restricted to the radial 
walls. Between the tracheids, there were wood rays. These 
were continuous in S. insigne, but were interrupted in S. 
plurifoliatum where they were represented only by a group 
of parenchyma cells in the angles between adjacent tracheids. 
Large stems had a considerable thickness of cork on the 
outside, formed from a deep-seated phellogen. 

The leaves of Sphenophyllum showed a wide range of 
structure, some being deeply cleft, while others were entire 
and deltoid (Figs. 15M-O); yet all received a single vascular 
bundle, which dichotomized very regularly within the 
lamina. Some species were markedly heterophyllous, as 
illustrated in Fig. 15K, and in these the deeply cleft leaves 
were usually near the base, while the entire ones were higher 
up on lateral branches, an arrangement that suggests that 
the former might represent juvenile fohage. 



SPHENOPSIDA 99 

A number of cones, referred to the genera Sphenophyllo- 
stachys or Bowmanites, have been found attached to the 
parent plant; others, found detached, are placed in these 
genera on the basis of their general similarity. A number of 
other genera of cones are also referred to the Sphenophy Hales, 
but on less secure grounds. Some of them represent the most 
complex cones in the whole plant kingdom. One of the 
earUest to appear in the fossil record is Eviostachya, des- 
cribed by Leclercq^*, from the Upper Devonian of Belgium. 
Less than 6 cm long and less than i cm in diameter, each cone 
had at its base a whorl of six bracts. Above this were whorls 
of sporangiophores, six in each whorl. Each sporangiophore 
was itself highly compUcated (Fig. 1 5G) and branched in a very 
characteristic way (Fig. 15H), bearing a total of twenty-seven 
sporangia in a reflexed position. Sporangiophores in successive 
whorls stood vertically above each other, as is characteristic of 
the Sphenophyllales, but there were no bracts between them. 

Cheirostrobus, from the Lower Carboniferous of Scotland, 
was a large cone, 3-5 cm across, and had thirty-six sporangi- 
phores in each whorl, subtended by the same number of 
bracts, each with bifurcated tips (Fig. 15I). The arrangement 
of the vascular supply to these appendages is interesting in 
that a common 'trunk-bundle' supplied three sporangio- 
phores and the three bracts subtending them. This has led 
some morphologists to suggest a more comphcated inter- 
pretation of the cone structure than is really necessary, based 
on the supposition that each trunk-bundle represented the 
vascular supply to one compound organ made up of three 
fertile leaflets and three sterile leaflets. 

Sphenophyllostachys fertilis { = Sphenophyllum fertile, 
= Bowmanites fertilis) from the Upper Carboniferous (Fig. 
15J) was also a complex cone. Up to 6 cm long and 2-5 cm 
in diameter, it was made up of whorls of superimposed 
sporangiophores, six in a whorl, each subtended by a pair 
of sterile appendages (possibly homologous with one bifid 
bract). Each sporangiophore terminated in a *mop' of 



100 THE MORPHOLOGY OF PTERIDOPHYTES 

branches, about sixteen in number, each bearing two re- 
flexed sporangia. Only detached cones have, so far, been 
found, but they are presumed to have belonged to some 
member of the Sphenophyllales, because of the triarch or 
hexarch arrangement of the primary wood in the axis. 

Sphenophyllostachys { = Bowmanites) Dawsoni, on the 
other hand, is known to have been borne on stems like those 
of Sphenophyllum plurifoliatum. The cone was up to 12 cm 
long and i cm in diameter and bore whorls of bracts, fused 
into a cup near the base, but with free distal portions. In the 
axils of these bracts, and fused with them to a certain extent 
(Fig. 15P), were branched sporangiophores. In one form 
(forma a) each sporangiophore had three branches arranged 
in a very characteristic way (Fig. 15Q), each terminating in 
a single reflexed sporangium. In another form (forma y), 
there were six branches. 

Sphenophyllostachys { = Bowmanites) Roemeri was similar 
in its organization to S. Dawsoni, forma a, except that each 
branch of the sporangiophores bore two reflexed sporangia 
(Fig. 15R). 

In recent years, a number of relatively simple cones have 
been described, which are nevertheless beheved to belong to 
the Sphenophyllales. Thus, in Bowmanites bifurcatus, each 
sporangiophore forked only once, while in Litostrobus 
iowensis the sporangia were borne singly on a short un- 
branched stalk. In the latter species, the sporangia were not 
reflexed but, despite this very simple organization, an 
affinity with Bowmanites is presumed, because of the 
number of bracts and the number of sporangia in a whorl 
(twelve and six respectively).^ The discovery of these simple 
cones has led to the suggestion that, within the Spheno- 
phyllales, evolution has involved progressive simplification. 

While the vast majority of the Sphenophyllales were 
homosporous, at least one, Bowmanites delectus, was hetero- 
sporous^ with megaspores about ten times the size of the 
microspores. 



SPHENOPSIDA lOI 

Calamitales 

This group reached the peak of its development in the Upper 
Carboniferous, when a large number of arborescent species 
was co-dominant with the Lepidodendrales in coal-measure 
swamp forests ; yet by the end of the Permian the group had 
become extinct. The first representatives to appear, in the 
Upper Devonian, were the Asterocalamitaceae, a group 
which differed from the later Calamitaceae in a number of 
interesting details. Asterocalamites { = Archaeocalamites) 
(Fig. i6A) had woody stems up to i6 cm in diameter, 
strongly grooved on the outside, with the grooves con- 
tinuing through successive nodes (i.e. not alternating). The 
leaves, up to lo cm long, were in whorls at the nodes and 
forked many times dichotomously, in a manner strongly 
reminiscent of Calamophyton leaves. At intervals along the 
more slender branches, there were fertile regions, in which 
there were superimposed whorls of peltate sporangio- 
phores, each bearing four reflexed sporangia (Fig. i6B). 
Sometimes the fertile regions were interrupted by a whorl 
of leaves, but these were apparently normal leaves and could 
not be regarded as bracts. The absence of any regular 
association between bracts and sporangiophores makes an 
interesting comparison with the cones of the later Calami- 
taceae. 

Protocalamites was one of the earhest representatives of 
the Calamitaceae, being present in the Lower Carboniferous 
rocks of Pettycur, Scotland. Its stems were ridged, with the 
ridges alternating in successive internodes, like those of 
most members of the family, but they differed in one im- 
portant respect. Examination of a transverse section of a 
petrified stem (Fig. i6C) reveals a marked development of 
centripetal wood, as well as centrifugal (i.e. the primary 
wood was mesarch). As in Catamites and in Equisetum, the 
protoxylem tended to break down to form a carinal canal. 
Secondary wood was laid down to the outside of the meta- 
xylem, but the primary wood-rays were so wide that it gives 







Fig. 16 

Asterocalamites {= Archaeocalamites) : A, stem and leaves; 
B, fertile region. Protocalamites : c, part of internodal vascular 
system. Protocalamostachys : d, sporangiophore. Calamites: 



SPHENOPSIDA 103 

the appearance of having been formed in separate strands, 
although in fact it was formed from a continuous vascular 
cambium. 

Protocalamostachys is the name given to a peculiar cone 
described by Walton^^^ from Lower Carboniferous rocks in 
the Island of Arran. Two small pieces of the cone had 
dropped into the hollow stump of a Lepidophloios before it 
became petrified. Unhke the cones of other members of the 
Calamitales, its sporangiophores branched twice (Fig. 16D), 
instead of being peltate. In this respect, it showed some 
affinities with the Sphenophyllales and also with the 
Hyeniales. However, Walton compares it most closely with 
Pothocites, a cone associated with leaves of the Astero- 
calamites type. Furthermore, within the axis of the cone 
there is centripetal primary wood as in the stem of Proto- 
calamites. 

The height to which Calamites grew is difficult to deter- 
mine, because of the fragmentary nature of the remains, but 
it is almost certain that some specimens must have attained 
a height of 30 m with hollow trunks whose internal diameter 
was up to 30 cm. Strictly speaking, the generic name 
Calamites should be applied only to pith casts of stems and 
branches, while petrified wood should be described under 



E, reconstruction of Eiicalamites type. Annularia: F. Astero- 
phyllites: G. Calamites {= Arthropitys): H, part of internodal 
vascular system. Palaeostachya: i, sporangiophore and bract. 
Calamostachys : J, part of cone in l.s. Eqiiisetum pratense: K, 
plant with young sterile shoot; l, young fertile shoot with cone; 
M, sporangiophore; n, spore with elaters; o, nodal vascular 
arrangement (1, internodal bundles; 2, leaf trace; 3, branch 
traces); p, antherozoid. Eqiiisetum sylvaticiim: Q, internodal 
vascular bundle (4, carinal canal with remains of protoxylem). 
Archegonium: r (E. hyemale). Embryo: s (E. arvense) (f, foot; 
1, leaf primordia; r, root primordium; x, stem apex) 

(a, after Stur; b, Renault; d, Walton; e, Hirmer; f, g, Abbott; 
I, Baxter; J, Scott; k, l, Duval-Jouve; n, Foster and Gifford; 
o, Fames; p, Sharp; r, Jeff'rey; s, Sadebeck) 



104 THE MORPHOLOGY OF PTERIDOPHYTES 

the form genus Arthropitys, but common usage has extended 
the apphcation of the name Calamites to include all methods 
of preservation. The pith casts exhibit ridges and grooves, 
corresponding in number to the protoxylem strands, run- 
ning up the inside of the secondary wood and alternating at 
successive nodes. In this respect they differ from Meso- 
calamites, in which there was some variabiUty from node to 
node, the ridges sometimes alternating and sometimes con- 
tinuing straight across the nodes. A number of subgenera 
are recognized which differed in their mode of branching 
and, hence, in their general form. The subgenus Eucalamites 
branched at every node. Fig. i6E is oi Eucalamites carinatus, 
in which there were only two branches at each node, but 
other species branched more profusely. By contrast, the sub- 
genus Stylocalamites branched only near the top of the erect 
organpipe-Hke trunk. 

Transverse sections of Calamites (Arthropitys) show very 
little primary wood indeed, for secondary thickening pro- 
vided most of the wood (Fig. i6H). The protoxylem was 
represented by carinal canals and the small amount of 
metaxylem present was entirely centrifugal. The wood rays 
varied, according to species, dividing the secondary wood 
into segments in some species, or losing their identity in a 
continuous cylinder of wood in others. In all cases the wood 
contained small wood-rays in addition, but otherwise was 
composed entirely of tracheids with scalariform pitting or 
with circular bordered pits on the radial walls. 

The leaves of Calamites were unbranched, with a single 
mid-vein, and occurred in whorls of four to sixty. In most 
species, they were free to the base, but in a few they showed 
some degree of fusion into a sheath. They are placed in one 
or other of two form genera, according to their overall 
shape, Annularia being spathulate or deltoid (Fig. i6F), while 
Asterophyllites were linear (Fig. i6G). The latter were 
pecuHar in being heavily cutinized, with the stomata res- 
tricted to the adaxial surface, suggesting that the branches 



SPHENOPSIDA 105 

bearing them may have been pendulous. It is probable, 
therefore, that the cones were pendulous, too. 

The cones of Calamites were borne in a variety of ways, 
in some species singly at the nodes, in others in terminal 
groups or infructescences or on specialized branches. ^ Many 
species are known, but most of them are placed in one or 
other of the two genera Calamostachys and Palaeostachya. 
As originally defined, these two genera were clearly distinct, 
but, in the light of many newly discovered species, Andrews^ 
has questioned whether the distinction is now justified. In 
both genera there were whorls of peltate sporangiophores 
bearing four reflexed sporangia (Figs. 1 61 and 1 6 J), alternating 
with whorls of bracts fused into a disc near their point of 
attachment. Whereas the sporangiophores were in vertical 
rows, the bracts in successive whorls alternated with one 
another. While the number of bracts in a whorl bore a 
definite relationship to the number of sporangiophores, the 
actual numbers varied from species to species and, some- 
times, from individual to individual. Calamostachys Binne- 
yana, a cone about 3-5 cm long and 7-5 mm wide, had six 
sporangiophores in each whorl and twelve bracts. C. 
magnae-crucis was more complicated, in having alternating 
vascular bundles in successive internodes within the cone 
and in having sporangiophores and bracts so numbered that, 
if 'n' were the number of vascular bundles, then the number 
of sporangiophores was in in each whorl and the number of 
bracts 3n; the number 'n' could be either seven or eight. 
Most species were homosporous, but some were definitely 
heterosporous. Thus, in C. casheana the megaspores were 
three or four times the size of the microspores, while in 
C americana they were about twice the size. 

Whereas the sporangiophores of Calamostachys stood out 
at right angles to the cone axis, those oi Palaeostachya stood 
out at an angle of about 45°, and in some species they appear 
to have been in the axil of the bract whorl below. P. vera had 
eight to ten sporangiophores in each whorl and twice as 



I06 THE MORPHOLOGY OF PTERIDOPHYTES 

many bracts (according to an early description by Williamson 
and Scott^^, but according to Hickling,*^ the same number). 
Perhaps the most interesting feature shown by this species is 
the course taken by the vascular bundle supplying the 
sporangiophore. As illustrated in Fig. i6I, it travelled up in 
the cortex of the cone axis to a point about midway between 
the bracts, and then turned downwards, before entering the 
stalk of the sporangiophore. To those morphologists who 
regard vascular systems as highly conservative, this impUes 
that Palaeostachya must have evolved from some ancestral 
form in which the sporangiophore stood midway between 
the bract whorls, as in Calamostachys, and that during the 
'phyletic sUde' the vascular supply had lagged behind. 
Palaeostachya Andrewsii showed the same feature, but in 
P. decacnema the sporangiophore bundle took a direct 
course. One concludes, therefore, that this last species is 
more advanced than the others in this respect. In P. 
Andrewsii, the numbers of sporangiophores and bracts in a 
whorl were twelve and twenty-four respectively, while in 
P. decacnema they were usually ten and twenty. 

The above brief review of the Calamitales brings out some 
interesting evolutionary trends, which are paralleled very 
closely in the Lepidodendrales. Thus, the production of 
increasing amounts of secondary wood was accompanied, 
in both groups, by a reduction of the primary wood, of 
which the centripetal metaxylem was the first to go, being 
replaced either by pith or by a central hollow. At the same 
time, there was a trend in the fertile regions from a 'Selago 
condition' to a compact cone, in which the sporangia were 
protected by overlapping sporophylls in one group and by 
bracts in the other. Then, having reached their zenith to- 
gether, both groups became extinct at about the same time. 

Equisetales 

The only representatives of the Sphenopsida that are alive 
today belong to the single genus Equisetum and, of this, only 



SPHENOPSIDA 107 

some twenty-five species are known. Eleven of them occur 
in the British Isles, where they are known as 'horse-tails'. 
The genus is distributed throughout the world with the 
exception of Australia and New Zealand, from which 
countries it is completely absent. All the species are her- 
baceous perennials, but there is an interesting range of 
growth habits, for some are evergreen, while others die back 
to the ground each year. Early statements that a Hmited 
amount of secondary thickening occurs are now discredited, 
for there is no evidence that a cambium is present in any 
species. Most species are, therefore, very hmited in size; the 
largest species, E. giganteum, has stems up to 13 m long, but 
since they are only 2 cm thick the plant depends on the sur- 
rounding vegetation for its support. The largest British 
species, E. telmateia, sometimes attains a height of 2m and is 
free-standing in sheltered locahties, but most species are 
much smaller than this and are between 10 and 60 cm tall. 
In all species there is a horizontal rhizome from which 
arise aerial stems that branch profusely in some species 
(e.g. Equisetum telmateia, E. arvense) or remain quite un- 
branched in others (e.g. E. hiemale). The leaves, in all species, 
are very small and are fused into a sheath, except for their 
extreme tips which form teeth round the margin of the 
sheath. They are usually without chlorophyll, photosyn- 
thesis being carried out entirely by the green stems. In the 
past, there have been discussions as to whether the small 
leaves of Equisetum represent a primitive or a derived con- 
dition, but, in the hght of the fossil record, it is now clear 
that they have been reduced from larger dichotomous struc- 
tures (i.e. that they are derived). The stems are ridged, each 
ridge corresponding to a leaf in the node above, and the 
ridges in successive internodes alternate with one another 
(as, of course, do the leaves in successive leaf-sheaths). 
There are, however, some departures from this regular 
alternation, as the number of leaves in a whorl diminishes 
from the base to the apex of the stem.^^ 



I08 THE MORPHOLOGY OF PTERIDOPHYTES 

The sporangia are borne in a cone, which in some species 
{Equisetum arvense) terminates a special aerial axis that 
lacks chlorophyll, is unbranched and appears before the 
photosynthetic axes. In other species the fertile shoot is 
green and may subsequently give rise to vegetative branches 
lower down (e.g. E. limosum and E. pratense), after the cone 
has withered. In yet other species, most of the lateral 
branches may terminate in a cone (e.g. the Mexican species 
E. myriochaetum). This last arrangement is commonly re- 
garded as the primitive condition, on the basis that it in- 
volves the least speciahzation, but it must be reahzed that 
real evidence for this view is lacking. 

The internal anatomy of the stem of Equisetum presents 
an interesting association of xeromorphic and hydromorphic 
characters, together with a vascular system which is without 
parallel in the plant kingdom today, and whose correct 
morphological interpretation has long been the subject of 
controversy. The ridges in the stem are composed of 
sclerenchymatous cells, whose thick walls are so heavily 
sihcified as to blunt the edge of the razor when cutting 
sections. Stomata are restricted to the 'valleys' between the 
ridges and are deeply sunken into pits whose openings may 
be partly covered by a flange of cuticle. The walls separating 
the guard cells from their accessory cells bear pecuHar comb- 
like thickenings which are known elsewhere only in the 
leaves of Calamites. Beneath each of the valleys is a 'vallecu- 
lar canal' and the central region of the internodes of aerial 
stems consists of a large space (but, in subterranean stems, 
the centre may be occupied by pith). At the nodes, there is a 
transverse diaphragm. Such an arrangement of air channels, 
together with a very reduced vascular tissue, are features 
normally found in water plants and contrast strikingly with 
the heavy cuticle, sunken stomata and reduced leaves. 

The internodal vascular bundles lie beneath the ridges of 
the stem and are quite characteristic (Fig. i6Q). As in 
Calamites, the protoxylem is endarch and is replaced by a 



SPHENOPSIDA 109 

carinal canal (4), in which may be seen lignified rings which 
are all that remain after the dissolution of annular tracheids. 
To the outside of each carinal canal, and on the same radius, 
lies an area of phloem, flanked on either side by a lateral 
xylem area. This lateral xylem may contain further proto- 
xylem tracheids with annular thickenings, but otherwise 
consists of metaxylem elements which may be tracheids 
with helical thickening, or with pits, or may even be true 
vessels. Two types of vessel element occur, one with simple 
perforation plates and the other with reticulate, but it must 
be emphasized that they are restricted to the internodes and 
that they seldom occur more than three in a row. They do 
not, therefore, form conducting channels of great length as 
do the vessels of flowering plants. ^^ In some species (e.g. 
Equisetum Utorale) each internodal bundle is surrounded by 
its own separate endodermis, in others (e.g. E. palustre) 
there is a single endodermis running round the stem outside 
all the bundles, while in yet other species (e.g. E. sylvaticum. 
Fig. 16Q) there are two endodermes, one outside and the 
other inside all the bundles. 

At the nodes (Fig. 16O) the vascular bundles (i) are 
connected by a continuous cylinder of xylem, from which the 
leaf traces (2) and branch traces (3) have their origin. 
Neither vallecular canals nor carinal canals are present in 
this region and there have been disagreements as to whether 
there is any protoxylem here either, but the most recent 
investigations confirm its presence as a constant feature. ^^ 
This disposes of the view, held by some, that the internodal 
bundles represent leaf traces extending down through the 
internode to the node below. An alternative view used to be 
held— that the vascular network represents a kind of dictyo- 
stele, in which the spaces between the internodal bundles 
represent leaf-gaps. However, this is unhkely, in view of the 
arrangement known to have existed in the earhest relatives 
of the genus, such as Asterocalamites, where there was no 
alternation at the nodes. Furthermore, this view overlooks 



no THE MORPHOLOGY OF PTERIDOPHYTES 

the peculiar way in which the internodes of Equisetum are 
formed from an intercalary meristem. If analogies are to be 
sought with other pteridophytes, then it is not the internode, 
but the node, which should be compared. The vascular 
structure of the node can best be looked upon as a meduUated 
protostele. The internodal spaces then appear as perfora- 
tions, albeit of a pecuhar (intercalary) origin. 

Growth at the stem apex takes place as a result of the 
activity of a single tetrahedral apical cell, daughter cells 
being cut off in turn from each of its three cutting faces. 
Despite the spiral sequence of such daughter cells, subse- 
quent growth results in a whorled arrangement and three 
daughter cells together give rise to all the tissues which make 
up a node and an internode. It is interesting that, in the 
first-formed stem of the young sporeUng, there are three 
leaves in each whorl, but, nevertheless, it is stated that their 
initiation is in no way determined by the position of the 
cutting faces of the apical cell. Each leaf primordium grows 
from a single tetrahedral apical cell, and in the angle be- 
tween the leaf sheath and the axis, but on radii between the 
leaves, lateral bud primordia arise, also with a single apical 
cell. The lateral bud primordia subsequently become buried 
by a fusion of the base of the leaf sheath with the axis, with 
the result that, when it grows, it has to burst through the 
leaf sheath, so giving the appearance of an endogenous 
origin. However, not all branch primordia do grow, for in 
species such as Equisetum hiemale, although present, they 
are inhibited from growing beyond the primordial stage, 
unless the main stem apex should be destroyed or damaged. 
Each branch primordium, besides bearing leaf primordia, 
also bears a root primordium which in aerial axes is also 
inhibited from growing further. In underground axes, how- 
ever, they are not inhibited in this way. It is interesting to 
note that the roots which are apparently borne on a horizon- 
tal rhizome are, in fact, borne by the axillary buds hidden 
with its leaf sheaths, and not directly upon it. 



SPHENOPSIDA III 

The root grows from an apical cell with four cutting faces, 
the outermost of which gives the root cap. It may be triarch, 
tetrarch or diarch in its vascular structure and there is 
usually just a single central metaxylem element. The stele is 
surrounded by a pericycle, whose cells correspond exactly in 
number and radial position with those of the endodermis, 
since they are formed by a perichnal division in a ring of 
common mother cells. This has led to the statement that the 
root has a double endodermis, but this is incorrect, since the 
cells of the inner ring are without Casparian strips, and must 
be regarded as pericycle. 

The cone (Fig. i6L) invariably terminates an axis, whether 
it be the main axis or a lateral one, and bears whorls of 
sporangiophores, without any bracts or other leaf-Hke 
appendages interposed, although there is a flange of tissue 
at the base of the cone called the 'collar'. Each sporangio- 
phore is a stalked peltate structure, bearing five to ten 
sporangia which, although having their origin on the outer 
surface, become carried round during growth into a reflexed 
position on the underside of the peltate head (Fig. i6M). 
Within the cone axis, the vascular system forms a very 
irregular anastomosing system, without discernible nodes 
and internodes, from which the sporangiophore traces 
depart without any regular association with the gaps. The 
sporangiophore trace branches within the peltate head and 
each branch terminates near a sporangium. 

The sporangium has its origin in a single epidermal cell, 
which divides perichnally into an inner and an outer cell. 
The inner cell gives rise to sporogenous tissue. The outer cell 
gives rise to further blocks of sporogenous tissue and also to 
the wall of the sporangium. Adjacent cells may also add to the 
sporogenous tissue. The sporangium may therefore be des- 
cribed as eusporangiate in the widest sense and at maturity 
the sporangium is several cells thick. The innermost wall 
cells break down to form a tapetum, as also do some of the 
spore mother cells, and the ripe sporangium is two cells 



112 THE MORPHOLOGY OF PTERIDOPHYTES 

thick, of which the outer layer shows a characteristic spiral 
thickening. 

Each spore, as it matures, has deposited round it four 
spathulate bands, which are free from the spore wall except 
at a common point of attachment (Fig. i6N). These are 
hygroscopic, coiling and uncoihng with changes in humidity, 
and are referred to as 'elaters', although what function they 
perform during dehiscence of the sporangium is not clear. 
McClean and Ivimey-Cook^^ have shown that a distribution 
curve of the size of spores in Equisetum arvense is a bi-modal 
one, suggesting that a shght degree of heterospory exists, the 
large spores being some 25 per cent larger than the small 
ones. Furthermore, the smaller spores give rise to small male 
prothalli, whereas the large spores produce hermaphrodite 
ones. Whether this represents the early stage of the evolution 
of heterospory, or the last stages of a reversion to homo- 
spory, cannot be determined but, in any case, Uttle out- 
breeding advantage is likely to accrue, because the elaters 
become so entangled as the spores are being released that 
they are usually distributed in groups. Reports of com- 
pletely dioecious prothalh have been pubHshed, but these are 
probably based on observations made at a single moment in 
time. The prothalh o^ Equisetum are long lived, and extended 
observations would probably show that any one prothallus 
has archegonia alone for a time and then antheridia alone, 
as has indeed been demonstrated in some species. Differences 
in nutrition can also influence the behaviour of the prothalh 
for, under favourable conditions, only male prothahi result. 
Further work on this fascinating subject is clearly necessary. 

The prothallus consists of a flat cushion of tissue, varying 
in size from i mm across to 3 cm in some tropical species. 
From the underside are produced abundant rhizoids, and 
from the upper side numerous irregular upright plates, or 
lobes, which are dark green and photosynthetic. Archegonia 
are formed in the tissue of the cushion between the aerial 
plates and ji& ve projecting necks of two or three tiers of 



,.^^ 



SPHENOPSIDA 113 

cells in four rows. There may be a single neck canal cell or 
there may be two boot-shaped cells, lying side by side, as 
illustrated in Fig. 16R. There is also a ventral canal cell. 
The antheridia are sunken in the tissue of the basal cushion, 
but may also occur on the aerial lobes. They are massive and 
give rise to large numbers of antherozoids, which are spirally 
coiled and multiflagellate (Fig. 16P). 

The first division of the zygote is in a plane more or less 
at right angles to the axis of the archegonium. No suspensor 
is formed and the embryo is exoscopic. Fig. 16S shows the 
spatial relationships of the stem apex (x), the first leaves (1), 
the root (r) and the foot (f), as described as long ago as 1878, 
but it is now becoming clear that the various parts of the 
embryo are not so constant in position and origin as was 
formerly thought. 

There can be little doubt that the Equisetales are related 
to the Calamitales, but it is most unhkely that they represent 
their direct descendants. Remains of herbaceous plants 
resembHng Equisetum are placed in the genus Equisetites, 
They are traceable right back through the Mesozoic to the 
Palaeozoic, where several species have been described from 
the Upper Carboniferous. The situation is thus closely 
comparable with Selaginella, whose herbaceous ancestors 
were living alongside the related arborescent Lepidoden- 
drales in Carboniferous times. 




Pteropsida 



Sporophyte with roots, stems and spirally arranged 
leaves (megaphylls) often markedly compound and 
described as 'fronds' (although some early mem- 
bers showed Uttle distinction between stem and 
frond). Protostehc, solenostehc or dictyostehc, 
sometimes polycyclic (rarely polysteUc). Some 
with limited secondary thickening. Sporangia 
thick- or thin-walled, homosporous or hetero- 
sporous, borne terminally on an axis or on the 
frond, where they may be marginal or superficial on 
the abaxial surface. Antherozoids multiflagellate. 

Some botanists widen the definition of the Pteropsida to 
include, not only the megaphyllous pteridophytes, but also 
the gymnosperms and angiosperms, on the supposition that 
all three groups are related. While this may well be so, it 
seems preferable to retain the distinction between pterido- 
phytes and seed-plants and to restrict the definition of the 
Pteropsida so as to exclude all but the ferns. Even so, the 
group is an enormous one, with over 9,000 species, and 
shows such a wide range of form and structure that it is 
almost impossible to name one character which is diagnostic 
of the group. The reader will have noticed that almost all of 
the characters Usted at the head of this chapter are qualified 
in some way. 

It will readily be appreciated that, in such a large group, 
the correct status of the various subdivisions is very largely 

114 



PTEROPSIDA 115 

a matter of personal preference. Accordingly, there are 
almost as many different ways of classifying the group as 
there are textbooks dealing with it, and this is particularly 
true of the fossil members of the group. 

At this point, only the major subdivisions are presented, 
the details being deferred until each subgroup is dealt with. 

A Primofilices* 

1 Cladoxylales* 

2 Coenopteridales* 

B Eusporangiatae 

1 Marattiales 

2 Ophioglossales 

C Osmundidae 
Osmundales 

D Leptosporangiatae 

1 Filicales 

2 Marsileales 

3 Salviniales 

Primofilices 

This is a remarkable group of plants that first appeared in 
the Middle Devonian and survived until the end of the 
Palaeozoic. As the name suggests, they were probably the 
ancestors of modern ferns. They may be classified as follows : 

1 Cladoxylales* 

Cladoxylaceae* Cladoxylon* (Hierogramma, 

Syncardia, Clepsydropsis) 
Pseudosporochnaceae * Pseudosporochnus* 

2 Coenopteridales* 

Zygopteridaceae* Austroclepis'^ , Metaclepsydropsis*, 

Diplolabis* Dineuron* 
Rhacophyton* Ankyropteris,* 
Etapteris* { = Zygopteris, 
= Botrychioxylon) Tubicaulis* 



tl6 THE MORPHOLOGY OF PTERIDOPHYTES 

Stauropteridaceae* Stauropteris* 
Botryopteridaceae * Bo try op teris * 

The Cladoxylales are a particularly interesting group, 
whose correct phylogeny has long been a matter of con- 
troversy. On the one hand, they show a number of features 
in common with the Psilophytales and, indeed, Pseudo- 
sporochnus has only recently^® been transferred from that 
group. On the other hand, they show features in common 
with the Coenopteridales, whose later representatives had 
already begun to look fern-hke before they became extinct. 
The group thus stands in an intermediate position which 
strongly suggests a genuine phylogenetic connection be- 
tween the two groups. 

Several species of Cladoxylon are known, of which the 
earhest is C. scoparium, and our knowledge of this is based 
on one specimen about 20 cm long from Middle Devonian 
rocks of Germany. According to the reconstruction of the 
plant by Kraiisel and Weyland^^ (Fig. 17 A), there was a 
main stem, about 1-5 cm in diameter, which branched rather 
irregularly. Some of the branches bore fan-shaped leaves 
(Fig. 17B) ranging in size from 5 mm to 18 mm long. Some 
leaves were much more deeply divided than others, but all 
showed a series of dichotomies. On some of the branches, 
the leaves were replaced by fertile appendages which were 
also fan-shaped, each segment terminating in a single 
sporangium (Fig. 17C). 

The vascular system was highly complex and was poly- 
stelic; each of the separate steles was deeply flanged; both 
scalariform and pitted tracheids were present in the xylem. 
Such complex vascular structure is characteristic of all the 
species of Cladoxylon and some had the additional comph- 
cation of secondary thickening. C. radiatum was similar to 
C scoparium in that all the xylem was primary, and Fig. 
17D illustrates the way in which several xylem flanges were 
involved in the origin of a branch trace system. It also 



PTEROPSIDA 117 

illustrates the 'islands of parenchyma', as seen in transverse 
section, which are a common feature of the Zygopteridaceae 
too. Another feature, shared with the Coenopteridales, is the 
presence of 'aphlebiae' at the base of the lateral branch (or 
petiole?). These were similar in position to the stipules of 
many flowering plants and received separate vascular 
bundles (i). 

Fig. 17E illustrates another type of stem structure, found 
in Cladoxylon taeniatum and several other species, in which 
each of the xylem strands has an outer region of radially 
arranged tracheids which are thought to have been formed 
from a cambium. The arrows in the figure indicate that three 
of the stem steles were involved in the origin of branch 
traces. Successive branches, petioles and pinnae of descend- 
ing order had progressively simpler vascular structures, 
without secondary wood, and are described under separate 
form-generic names. Thus, Fig. 17F shows the Hierogramma 
type of stelar arrangement, in which there were six xylem 
regions, each with islands of parenchyma. Lateral branches 
from this had four xylem areas and are known as Syncardia 
(Fig. 17G). Clepsydropsis (Fig. 17H) was probably the next 
type of branch, or petiole, although there has been some dis- 
agreement among palaeobotanists about this. Its stele, as 
seen in transverse section, had the shape of an hour-glass 
(hence the generic name) and from it lateral pinna traces 
were given off" alternately, along with a pair of aphlebia traces 
(i). It should be noted that similar clepsydroid steles are 
known from a number of plants belonging to the Coenop- 
teridales. 

Pseudosporochnus is represented in Middle Devonian 
rocks of Germany, Scotland, Scandinavia and North 
America, but our knowledge of its morphology is based 
chiefly on the German species P. Krejcii (Fig. 17 1). It had an 
erect stem with a swollen base and a bushy crown of branches 
which forked dichotomously and terminated in small 
sporangia. According to Kraiisel and Weyland^^ there were 




Fig. 17 

Cladoxylon: a, reconstruction of S. scoparium; b, leaf; c, fertile 
appendages; d, origin of branch traces in C. radiatiim; e, C. 
taeniatum, portion of stem near origin of branch trace; f, 



PTEROPSIDA 119 

no organs that could be called leaves and, for this reason, 
the plant was placed in the Psilophytales. However, Leclercq 
has recently discovered abundant remains of the plant and 
investigations by her and Banks show that there are, in 
fact, spirally arranged leaves with ^several successive pairs 
of opposite divisions ; each of the numerous divisions then 
divides by several successive dichotomies ; the whole appen- 
dage is arranged in three dimensions'. Some of the leaves 
were fertile and their ultimate divisions terminated in a pair 
of sporangia. These workers were, furthermore, able to 
observe the vascular system of the plant and found that it 
had a very complex stellate form, as seen in transverse 
section. Their conclusion, published in a preliminary note,^® 
is that the genus has more in common with the Cladoxylaceae 
than with any other group of plants and that it should cer- 
tainly be removed from the Psilophytales. 

Coenopteridales 

This early group of ferns is a large one, consisting of many 
genera and species. It showed a wide range of growth habit, 
for some had creeping stems, others had erect trunks and 
yet others were epiphytes. As with members of the Cladoxy- 
lales, here too there is the problem of distinguishing leaf 



Hierogramma type of stele; G, Syncardia type of stele; h, Clep- 
sydropsis XyxiQ of siQlQ. Pseudosporochnus : i, reconstruction of 
P. Krejcii. Metaclepsydropsis duplex: J, stem stele; k, petiolar 
bundle near base ; l, petiolar bundle showing origin of laterals ; 
M, model showing petiolar bundle giving off laterals. Ankyrop- 
teris: N, A. Grayii, stem stele and origin of petiolar bundle; o, 
A. westfaliensis, petiolar bundle. Etapteris: p, E. Scottii, petiolar 
bundle and origin of laterals ; Q, E. Lacattei, reconstruction of 
sterile frond; r, E. Lacattei, reconstruction of fertile frond; 
s, E. Lacattei, sporangia 

(1, aphlebia traces; 2, peripheral loop; 3, branch trace) 

(a-c, I, after Krausel and Weyland; d-h, p, Bertrand; j-l, 
Gordon; n, Scott; q, r, Hirmer; s, Renault) 



120 THE MORPHOLOGY OF PTERIDOPHYTES 

from stem and the term 'Phyllophore' is sometimes used 
for intermediate orders of branching. 

Austroclepsis, occurring in Lower Carboniferous rocks of 
AustraUa, was first described^® as a species of Clepsydropsis, 
on account of its clepsydroid petioles. However, the mode of 
growth of the plant and the internal anatomy of the stem 
show that it was not a member of the Cladoxylales. It had a 
stout trunk, at least 30 cm in diameter and 3 m high, that 
must have looked superficially like modern tree ferns, but it 
differed fundamentally from these in that, within the mass of 
roots constituting the main bulk of the trunk, there were 
several stems instead of just one. These branched within the 
trunk and gave off numerous petioles in a 2/5 phyllotactic 
sequence and these, too, continued to run up within the 
trunk. Each of the many stems had a single stele, usually 
pentarch, in which there was a central stellate region of 
mixed pith surrounded by a zone of tracheids. The petioles 
had a rather narrow clepsydroid stele with two islands of 
parenchyma bounded by ^peripheral loops' of xylem, and it 
was from these peripheral loops that pinna traces were given 
off from alternate sides at distant intervals, each associated 
with aphlebiae. 

Metaclepsydropsis duplex, from Lower Carboniferous 
rocks of Pettycur, Scotland, ^^ had a creeping dichotomous 
stem, from which erect *fronds' arose at intervals. Its stele 
was circular in cross section or (just before a dichotomy) oval 
(Fig. 17J), with an inner region of mixed pith and an outer 
zone of large tracheids. The only protoxylem present was 
that associated with the origin of a leaf trace, there being no 
cauHne protoxylem at all. The leaf trace was at first oval in 
cross section (Fig. 17K) but soon became clepsydroid (Fig. 
17L). Pinnae were borne in alternate pairs, along with 
aphlebiae (i). In giving rise to a pair of pinna traces, the 
peripheral loop (2) became detached and then split into 
two (3). A new peripheral loop then quickly re-formed. 

Diplolahis Roemeri occurs in the same rocks and was very 



PTEROPSIDA 121 

similar to Metaclepsydropsis. Its stem anatomy differed, 
however, in that the inner region of xylem consisted entirely 
of tracheids. The tracheids of the outer region were arranged 
in radial rows, but are nevertheless beheved by some to have 
been primary in origin. The petiolar trace had a very narrow 
'waist', with the result that it appeared X-shaped in cross 
section. 

Dineuron ellipticum, also from Pettycur, on the other hand 
had no *waist' at all in the petiolar trace, which was elhptical 
in cross section. 

In all three of these Lower Carboniferous genera, the 
origin of pinna traces was the same, suggesting that pairs of 
lateral pinnae were arranged alternately along the petiole, 
or phyllophore. The frond was thus a highly compound one 
whose components formed a three-dimensional structure. 
However, it had not been realized just how complex they 
were until the important discovery, in 195 1, of a mummified 
specimen of Rhacophyton zygopteroides.^^ That this plant 
belonged to the Zygopteridaceae was established by examin- 
ing its internal anatomy. It had a fairly stout stem bearing 
roots and spirally arranged fronds. The lowermost fronds 
were sterile and were bipinnate, the pinnules consisting of 
dichotomous branchlets, apparently without any flattening 
to form a lamina. The fertile fronds were much larger and 
more complex, and had pairs of pinnae arranged alternately, 
just as had been deduced for Metaclepsydropsis from a study 
of petrified material. Each of the paired pinnae was similar 
in its branching to a sterile frond. Whereas the lower pinna 
pairs had branched aphlebiae, these were replaced in the 
higher pinna pairs by profusely branched structures bearing 
numerous terminal sporangia. These were about 2 mm long 
and were without any specially thickened annulus. 

At least eight species are known of the Upper Carbonifer- 
ous genus Ankyropteris, which derives its name from the 
fact that the petiolar trace in some species, e.g. A. westfali- 
ensis, was shaped hke a double anchor (Fig. 17O). In some 



122 THE MORPHOLOGY OF PTERIDOPHYTES 

Other species the petiolar trace was much less extreme, 
having less 'waist', but all were ahke in that the islands of 
parenchyma were much extended tangentially and in that 
the peripheral loop remained closed throughout the origin 
of a pinna trace. Another pecuhar feature was that the pinna 
trace was undivided, suggesting that the pinnae were single, 
instead of paired as in most other members of the group. A. 
Grayi, from British coal measures, had a stem of considerable 
length which was over 2 cm in diameter. It was probably a 
climbing plant. The petioles were borne in a 2/5 phyllotactic 
spiral, corresponding with the five rays of the stellate stele 
(Fig. 17N). As in other members of the group, there were 
two distinct regions in the stele, but the inner region showed 
a clear distinction between a zone of tracheids and a 
central pith, while the outer region showed no evidence of 
radial arrangement at all and was clearly primary in origin. 
The petiolar traces of Etapteris were pecuhar in that the 
'peripheral loops' remained open throughout (i.e. there were 
no loops at all). Two pinna traces became detached, fused 
and then separated again, before passing out into the paired 
pinnae (Fig. 17P). The Permian species, E. Lacattei, is inter- 
esting in having progressed further than other members of 
the group in the evolution of a photosynthetic lamina, for 
the ultimate pinnules were flattened (Fig. 17Q). In the fertile 
regions of the frond (Fig. 17R) the pinnules were replaced 
by groups of sporangia. These were club-shaped, slightly 
curved, and had a distinct broad annulus of thickened cells 
(Fig. 17S). Some Etapteris fronds were attached to trailing 
stems, while others belonged to tree-ferns with stout trunks. 
The nomenclature of the latter is, however, rather trouble- 
some. The names Zygopteris and Botrychioxylon which have 
been used are probably synonymous. Z. primaria had a 
trunk about 20 cm in diameter, most of which consisted of a 
tangle of rootlets and leaf bases. In the centre was a single 
stem 1-5 cm across, with a five-rayed stele showing the usual 
two regions, but in this case the outer region looks very much 



PTEROPSIDA 123 

as if it had been formed from a cambium, and many mor- 
phologists describe it as secondary wood. Botrychioxylon 
paradoxum had a very similar appearance, but in this stem 
the cells of the inner cortex were also regularly arranged in 
radial rows. It would seem, therefore, that the whole of the 
growing point of the stem must have been organized in a 
pecuharly regular manner and that great caution should be 
used in describing even the outer xylem as secondary. 

Some species of Tubicaulis were tree ferns, while others 
were epiphytes. They are characterized by a frond form 
which approached closely to that of a present-day fern, for 
the fronds coming off in spiral sequence from the stem were 
pinnate and the pinnae were arranged in one plane. 

Stauropteris is represented by two species, S. burntislan- 
dica from the Lower Carboniferous and S. oldhamia from 
the Upper Carboniferous. Although the method of branching 
of the frond was similar to that of many of the Zygopteri- 
dales, differences in the vascular system are sufficient to 
warrant the creation of a separate family. The most import- 
ant of these is the absence of islands of parenchyma in the 
xylem of the petiolar traces. It is beheved that, so far, only 
portions of fronds have been found and that the stems have 
yet to be discovered. Fig. 18A shows how the frond of S. 
burntislandica was constructed, pairs of pinnae arising alter- 
nately along the petiole, each associated with aphlebiae. 
Then each pinna gave rise to secondary pinnae in the same 
way and this pattern was repeated at all levels of branching 
within the frond. The vascular system of the petiole of S. 
oldhamia (Fig. 18B) consisted of four regions of xylem either 
contiguous or separate from each other, each with a mesarch 
protoxylem. The smaller branches, however, tended to have 
a single tetrarch strand. 

Perhaps the most interesting feature of all about Stauro- 
pteris burntislandica is the fact that it was heterosporous. 
Its megasporangia (Fig. 18C), when found isolated, are 
called Bensonites fusiformis. They were strangely fleshy at 



124 THE MORPHOLOGY OF PTERIDOPHYTES 

the base and most commonly contained two functional 
megaspores along with two very small and, presumably, 
abortive ones, although examples have been found with 
four, six or eight megaspores. It is believed that the whole 
structure was shed from the parent plant without prior 
dehiscence. The microsporangia of S. oldhamia (Fig. i8D) 
were spherical, were typically eusporangiate in having a 
thick wall and had a terminal stomium where dehiscence 
took place, but there was no annulus of thick-walled 
cells. 

In the past, the Botryopteridaceae were often described as 
much simpler in their organization than the rest of the 
Coenopteridales, but recent investigations on both sides of 
the Atlantic have demonstrated that this is far from the case. 
Botryopteris antiqua, from the Lower Carboniferous of 
Scotland, is the earliest known species and was also the 
simplest in its internal anatomy. It had traihng dorsiventral 
axes up to 2 mm in diameter, which gave rise to erect, or 
semi-erect, radial stems bearing petioles, in spiral succession, 
and roots. The petioles then underwent branching, of up to 
five successive orders, to produce a multipinnate branch 
system. There was no flattening of the pinnules to form a 
lamina anywhere in the frond of this species and the distinc- 
tion between stem and petiole is purely arbitrary. The three 
types of stele are illustrated in Fig. i8E, where '2' indicates 
the one belonging to the trailing dorsiventral axis. It was a 
soUd rod of tracheids with multiseriate pits or with scalariform 
or reticulate pits. The single protoxylem group was lateral 
and almost, but not quite, exarch. The radial stems were 
about the same diameter, but the stele was circular in cross 
section with the smallest tracheids (protoxylem?) in the centre 
(3). The petioles were somewhat smaller, up to 1-4 mm in 
diameter, and had an oval stele with a lateral protoxylem (4). 
As the branches of the frond divided and sub-divided, the 
stele became smaller and smaller until the ultimate pinnules 
had only a few tracheids or even only one. The sporangia 




Fig. 18 

Stauropteris burntislandica : a, reconstruction of part of frond; 
c, megasporangium {=Bensonites fiisiformis). Stauropteris 
oldhamia: b, vascular system; d,. microsporangium. Botryopteris : 
E, vascular strands of B. antiqua; f, petiolar strand of B. ramosa; 
G, petiolar strand of B.forensis; h, vascular system of ^. trisecta; 
I, reconstruction of part of the frond of an advanced species; 
J, sporangium of B. globosa. 

(1, aphlebia traces; 2, dorsi ventral stele; 3, radial stele; 4, 
dorsiventral petiole trace) 

(a, c, e, after Surange; b, g, Bertrand; d, f, Scott; h, Andrews; 
I, Delevoryas and Morgan; J, Murdy and Andrews) 

were globose, up to 0-25 mm across, and had a multicellular 
annulus that occupied almost half the surface area. 

A comparison of this early species with those of the Upper 
Carboniferous and the Permian shows that there was a 
trend in the evolution of the petiole trace towards a greater 
degree of dorsiventrahty, together with an increase in the 
number of protoxylems. Thus, Botryopteris ramosa (Upper 

125 



126 THE MORPHOLOGY OF PTERIDOPHYTES 

Carboniferous) had a shallow gutter-shaped stele with three 
protoxylems (Fig. i8F), whereas B.forensis (Permian) had a 
stele shaped like the Greek letter oj in transverse section, with 
up to fifteen protoxylems (Fig. i8G). Some of these later 
species, furthermore, are known to have had laminate 
pinnules (Fig. i8I). 

The complexity of the branching of the later species of 
Botryopteris is illustrated by the reconstruction of the stelar 
system ofB. trisecta (Fig. i8H). Its erect stem had a cyhndri- 
cal protostele and bore leaves in a spiral sequence. The 
petioles had an oval vascular strand and branched into three. 
The two lateral branches then trisected again but, whereas 
the median traces in each case were co shaped, the lateral 
ones were cylindrical, Uke the stem stele. The whole frond 
was arranged in three dimensions, except for the ultimate 
pinnules which were disposed in one plane. 

Associated with this plant were found some remarkable 
spherical masses, containing thousands of sporangia, which 
are believed to represent the fertile parts of the frond, al- 
though in the meantime they are described under a separate 
specific name, Botryopteris globosa. The whole mass was up 
to 5 cm across and had, running through it, a system of 
branches with w shaped steles, but how it was connected to 
the parent plant is not known. Each sporangium was pear- 
shaped (Fig. 1 8 J) and the distal half consisted entirely of 
thick- walled cells, except for a stomium of thin-walled cells 
over the apex. In most species of Botryopteris, the sporan- 
gium wall is described as only one cell thick, suggesting that, 
in this respect at least, they were leptosporangiate. It is 
apparently true of some of the sporangia of B. globosa, but 
not of all, for some clearly had a second layer of thin-walled 
cells on the inside. This may well have shrivelled after the 
spores had been shed, so becoming invisible when petrified. 
Thus, although approaching the leptosporangiate condition, 
B. globosa had certainly not yet achieved it, and the same is 
probably true of all the species. 



PTEROPSIDA 127 

Eusporangiatae 

Marattiales 

Asterothecaceae* Psaronius* Asterotheca,^ 

Scolecopteris,* Acitheca,"^ 

Eoangiopteris* 
Angiopteridaceae Angiopteris 
Marattiaceae Marattia 
Danaeaceae Danaea 
Christenseniaceae Christensenia 

Ophioglossales 
Ophioglossaceae Ophiglossum Botrychium, 

Helmin thostachys 

Marattiales 

It was customary in the past to describe the Carboniferous 
as the Age of Ferns. This was because of the abundance of 
large fern-like fronds in the coal-measures, but it is now 
known that many of them really belonged to gymnosperms, 
for they have been found in association with seeds. Indeed, 
it is now suspected that most of them were gymnospermous. 
However, there can be no certainty about sterile fronds and 
these must, therefore, be placed in a number of form genera 
defined on the basis of the overall shape of the frond and on 
the shape and venation of the pinnules. Pecopteris is one of 
these and a large number of species are known. Some of 
them were certainly gymnosperms, but others were equally 
certainly ferns, for they bore sori of thick-walled sporangia. 
The frond, sometimes as much as 3 m long, was many times 
pinnate and the pinnules were attached along their entire 
base, each with a single midrib. The lateral veins were some- 
what sparse and branched dichotomously once or twice 
(Figs. 19D and 19E) or remained unbranched. 

Asterotheca is the name given to pecopterid fronds bearing 
sessile sori made up of four or five sporangia fused at the 
base into a synangium, but with the distal part free (Fig. 




Fig. 19 

of E. Andrewsii. Acitheca: c, sorus of A. ^^^>''"^/^^^',,^' 'f 5 ^ 
pinnae. Asterotheca: e, fertile pinnae of A. Candolleam, f. 



PTEROPSIDA 129 

19F). The sori were commonly arranged in two series along 
the pinna, as illustrated in Fig. 19E, each associated with a 
veinlet in the lamina. Scolecopteris was similar, except that 
the sorus was elevated on a short pedicel, or receptacle 
(Fig. 1 9 A). In Acitheca, the sporangia were elongated and 
pointed and were arranged round a central plug-hke recep- 
tacle (Fig. 19C). Eoangiopteris is regarded as a more advanced 
type of sorus^^ since it was Hnear instead of radial. Each had 
a cushion-like receptacle, on which were five to eight 
sporangia (Fig. 19B). In all these genera, the sporangium 
wall was very massive, many cells thick, and the number of 
spores Hberated from each was very high, e.g. up to 2,000 
in Aster otheca parallela. 

Fronds of these various types are often found in associa- 
tion with stout trunks that bore a superficial resemblance to 
those of modern tree-ferns, some of them as much as 15 m 
high, but organic connection between fertile frond and 
trunk has not yet been demonstrated conclusively. The 
evidence, nevertheless, suggests that Asterotheca fronds were 
borne in a crown at the summit of trunks known as 
Psaronius. Many species, belonging to a number of sub- 
genera of Psaronius, are known and most of them had 
remarkably complex stelar anatomy. Some of them were as 
much as 75 cm across, but most of this width was occupied 
by a thick mantle of roots, for the single stem in the centre was 

sorus. Angiopteris : G, h, sorus of A. crassipes; o, s, vascular 
system oiA. evecta ; w, young embryo ; x, older embryo. Marat tia : 
I, J, sorus ofM.fraxinea; Q, pinna; t, prothallus of M. Douglasii; 
u, archegonium. Danaea: k, l, sorus of D. elliptica; r, pinna; 
Y, embryo. Christensenia: m, n, sorus of C. aesculifolia ; p, 
pinna ; v, rhizome with leaf base 

(1, stipule; 2, stipular flange ; 3, flange of lamina; f, foot; 1, leaf; 
r, root; s, suspensor; x, stem apex) 

(a, b, f, after Mamay; c, Scott; d, Brogniart; e, Hirmer; g-n, 
p-R, Bower; o, s, Shove; t, u, y, Campbell; v, Gwynne-Vaughan ; 
X, Farmer) 
£ 



130 THE MORPHOLOGY OF PTERIDOPHYTES 

only a few cm in diameter. The stele of most species was a 
polycyclic dictyostele which, in the more complex types, 
contained as many as eleven interconnecting coaxial 
cylinders (or, rather, inverted cones fitting inside one 
another). Each was dissected into a number of mesarch 
meristeles completely surrounded by phloem, and the leaf 
traces at any particular level arose from the outermost 
system, while the inner systems were concerned with the 
origin of leaf traces at higher levels. The earhest examples, 
however, were simpler than this in their internal anatomy, 
e.g. P. Renaultii from the Lower Coal Measures had an 
endarch solenostele; and there is evidence that even the 
complex Permian species had a relatively simple structure 
near the base of the trunk, as would be expected by analogy 
with present-day ferns. Although the trunks were widest at 
the base, this was not because the stem within was wider 
but because there were a greater number of rootlets in the 
mantle; the stem v/as actually smaller towards the base. 
Some species had the leaves arranged distichously, some in 
three or four vertical rows, while others had them arranged 
spirally, as in most modern representatives of the group. 

The Marattiales are represented at the present day by 
about 200 species, placed in six (or seven) genera, most of 
which are confined to the tropics. Angiopteris (lOO species) 
is a genus of the Old World, extending from Polynesia to 
Madagascar, while Danaea (thirty-two species) is confined 
to the New World. Marattia (sixty species) is pan-tropical 
and extends as far south as New Zealand. Christensenia 
{ = Kaulfussia) is monotypic and is confined to the Indo- 
Malayan region. Most species have massive erect axes, but 
they never attain the dimensions of the fossil Psaronius. The 
largest, although reaching a diameter of i m, seldom exceed 
this in height. Christensenia and some species of Danaea, 
however, have creeping horizontal axes. The fronds of some 
species are larger than in any other living ferns and may be 
as much as 6 m long, with petioles 6 cm in diameter. They 



PTEROPSIDA 131 

may be as much as five times pinnately compound or, in 
some species, only once pinnate, like a Cycad leaf, while 
a few species have a simple broad lamina. Christensenia is 
pecuhar in having a palmately compound frond, as the 
specific name, C. aesculifolia, imphes. It is also pecuhar in 
having reticulate venation, for all the other genera have 
open dichotomous venation. All show circinate vernation, 
i.e. the young frond is coiled Hke a crozier and gradually 
uncoils as it grows. This is a feature which they share with 
Leptosporangiate ferns but which is absent from the 
Ophioglossales. With the exception of Danaea trichoman- 
oides, all the Uving members of the group have very leathery 
pinnules in whose ontogeny several rows of marginal initials 
are active (instead of a single row of marginal initial cells, 
as is more usual in leaves of other plants). In many 
species there are swelhngs, or pulvini, at the base of the 
pinnae and pinnules, which play a part in the geotrophic 
responses of the leaf, and in all species there are thick fleshy 
stipular flanges at the base of the petiole. Fig. 19V illustrates 
the appearance of the growing point of Christensenia, show- 
ing how the stipules (i) are joined by a commissure (2), 
and how they are folded over the primordium. After the 
frond has died and has been shed, the stipules and the leaf 
base remain attached to the axis and contribute much to its 
overall diameter. 

The young parts of Marattia and Angiopteris are covered 
with short simple hairs, while those of Christensenia 2ind 
Danaea bear peltate scales. Bower^ suggested that the nature 
of the dermal appendages in ferns can be a useful indicator 
of primitiveness or advancement, hairs being more primitive 
than scales; on this basis, therefore, Christensenia is rela- 
tively advanced and this conclusion is supported by its 
possessing reticulate venation. A comparison of fossil and 
recent members of the group suggests that there has been 
progressive reduction in height, from the tree-hke Carbon- 
iferous forms, through an intermediate stumpy erect axis, to 



132 THE MORPHOLOGY OF PTERIDOPHYTES 

an oblique or horizontal creeping rhizome; and on this basis, 
too, Christensenia along with Danaea is to be regarded as 
relatively advanced. 

The stem grows by means of a bulky type of meristem, not 
referable to a single initial celP and is characterized by the 
absence of sclerenchyma. Mucilage canals and tannin cells 
are abundant throughout and give the tissues a very sappy 
texture. The vascular anatomy of the stem is the most 
complex of all living pteridophytes and is surpassed in 
complexity only by fossil members of the group, such as 
Psaronius. A transverse section of the stem of Angiopteris 
(Fig. 19O) reveals a number of concentric rings of meristeles 
which, in a dissection (Fig. 19S), are seen to be part of a 
series of complex and irregular meshworks lying one within 
the other, yet interconnected by 'reparatory strands'. The 
whole system may be described as a highly dissected poly- 
cyclic dictyostele, but can best be visuahzed as a series of 
inverted cones of lace stacked inside each other. Although 
each meristele in the sporeUng is surrounded by an endo- 
dermis, in the adult state the endodermis is completely 
lacking. 

The earliest protoxylem elements to Hgnify are 'annular- 
reticulate', i.e. adjacent rings of lignin are interconnected by 
a network of strands, whereas later ones are reticulate. The 
metaxylem elements are scalariform and, in Angiopteris, the 
orientation of the elongated bordered pits is sometimes 
longitudinal, instead of transverse. This pecuhar arrange- 
ment has been called 'ob-scalariform'^^ and occurs else- 
where in the Ophioglossaceae and a few leptosporangiate 
ferns {Dennstaedtia and Blechnum). 

Each leaf, in a mature plant, receives a number of traces 
which arise from the outermost system of meristeles (the cut 
ends of the leaf traces are represented in black in Fig. 19S), 
but the root traces may arise from the innermost regions of 
the stele, threading their way through successive cones on 
their way to the cortex (cross-hatched in Fig. 19O). In those 



PTEROPSIDA 133 

Species with erect axes, the roots may emerge from the 
cortex some distance above the ground, so forming prop- 
roots. They are polyarch, with as many as nineteen exarch 
protoxylems and, while the aerial portions are medullated, 
as soon as the roots penetrate the soil the xylem extends 
right to the centre. Those of young plants usually contain 
a mycorrhizal fungus within the cortex (an oomycete known 
as Stigeosporium marattiacearun). 

In all genera, the sori are borne in a *superficiar manner, 
i.e. on the dorsal surface of the lamina, and beneath a vein 
or a veinlet. Christensenia has circular sori irregularly dis- 
tributed between the main veins (Fig. i9P)but, in all other 
genera, the sorus is more or less elongated beneath a lateral 
vein (Figs. 19Q and 19R). In Angiopteris the sporangia are 
free from each other (Figs. 19G and 19H), but in Marattia, 
Danaea and Christensenia they are fused into a synangium 
(Figs. 19I-N). Danaea is peculiar in having fleshy flanges 
of tissue (3) projecting between the adjacent synangia (or, 
according to some, in having the synangia sunken into a 
very fleshy pinnule). 

The first stage in the development of a sporangium is a 
perichnal division of a single epidermal cell, of which the 
inner half gives rise ultimately to the archesporial tissue, 
while the outer half gives rise to part of the sporangium wall, 
the rest of the wall being produced by the activity of 
adjacent cells. At maturity, the sporangium wall is many 
cells thick and there is a tapetum formed from the innermost 
wall cells. The occurrence of numerous stomata in the 
sporangium wall is an interesting feature rarely found else- 
where and presumably associated with its massive structure. 
Very large numbers of spores are produced from each 
sporangium (e.g. 1,440 in Angiopteris, 2,500 in Marattia and 
over 7,000 in Christensenia) and, since all the sporangia 
within a sorus mature and dehisce simultaneously, prodigious 
numbers of spores are shed. 

In those species with free sporangia, e.g. Angiopteris, there 



134 THE MORPHOLOGY OF PTERIDOPHYTES 

is a crude kind of annulus of thickened cells, whose contrac- 
tions pull the sides of the sporangium apart along a line of 
dehiscence on the inner face (Fig. 19H). Those with synangia 
have no such device ; instead, a thin part of the sporangium 
wall dries and shrinks to form a pore through which the 
spores can fall (Figs. 19K-N). The whole sorus in Marattia 
is very woody and, when ripe, splits into two halves which 
are slowly pulled apart, so as to expose the pores in each 
sporangium (Fig. 19J). 

Germination of the spores is rapid, occurring within a few 
days of being shed, and they develop directly into a massive 
dark green thalloid prothallus, which is mycorrhizal and is 
capable of living for several years. An old prothallus may 
be several centimetres long and may resemble closely a 
large thalloid liverwort (Fig. 19T). The prothallus is mon- 
oecious but, while the antheridia occur on both the upper and 
lower surface, the archegonia are confined to the lower 
surface, where they occur on the central cushion along with 
rhizoids. Both types of gametangia are sunken beneath the 
surface of the prothallus and the antheridium is large and 
massive. The archegonium (Fig. 19U) has a large ventral 
canal cell (except in Danaea) and a neck canal cell with two 
nuclei. The antherozoids are coiled and multiflagellate, as 
in other ferns. 

The first division of the zygote is at right angles to the 
axis of the archegonium, and the embryo is endoscopic. 
Thus, since the archegonial neck is directed downwards, the 
embryo is orientated with its shoot uppermost and, as it 
grows upwards, it bursts its way through the tissues of the 
prothallus. A minute suspensor is present in Danaea (Fig. 
19Y) and in some species of Angiopteris, but Marattia, 
Christensenia and most species of Angiopteris are com- 
pletely without a suspensor. This lack of constancy is 
paralleled in the Ophioglossales and has led to speculation 
as to its phylogenetic implications. A suspensor is generally 
held to be a primitive character and its presence even if not 



PTEROPSIDA 135 

universal in the Eusporangiatae, places them at a lower 
level of evolution than the remaining ferns, from which it is 
completely absent. 

The epibasal hemisphere gives rise to the shoot apex (x) 
and the first leaf (1) (Fig. 19W), but there is no regular 
pattern of cell divisions and the hypobasal region gives rise 
to a poorly developed foot (f) and, somewhat later, to the 
first root (r) (Fig. 19X). 

Chromosome counts give a haploid number n = 40 in 
Angiopteris. 

Ophioglossales 

This group of plants, completely without any early fossil 
record, is represented by about eighty living species, belonging 
to three genera. Botrychhim (thirty-five species) is cosmo- 
politan in distribution and Ophioglossum (forty-five species) 
is nearly so, but Helminthostachys (monotypic) is restricted 
to Indo-Malaysia and Polynesia. Two species are fairly 
common in the British Isles, Botrychium Iwiaria, 'Moon- 
wort' (Fig. 20A) which grows in dry grassland and on rocky 
ledges, and Ophioglossum vulgatum, 'Adder's Tongue' (Fig. 
20G) in damp grassland, fens and dune-slacks, while a third 
species, O. lusitanicum, is restricted to grassy cUff tops in 
the Channel Islands and the Scilly Isles. 

The stem, in most species, is very short and is erect, 
except in a few epiphytic species of Ophioglossum and in 
Helminthostachys, where it becomes a horizontal rhizome 
as the plant grows larger. Where the stem is erect, the leaves 
arise in a spiral sequence, but in temperate regions it is 
normal for only one leaf to be produced each year. In 
Helminthostachys, the leaves are borne in two ranks along 
the rhizome ; they are large and ternately compound, but in 
the other two genera they are usually much smaller. Those of 
Botrychium are pinnately compound ; those of Ophioglossum 
are simple or lobed and, unhke those of the other two 
genera, have a reticulate venation. At the base of the petiole 



136 THE MORPHOLOGY OF PTERIDOPHYTES 

there is a pair of thin stipules which enclose the apical bud; 
and the next leaf, when it begins to grow, has to break its 
way through the thin sheath covering it. UnUke all other 
living ferns their leaves are not circinately coiled when young. 
In all three genera, the fertile fronds have two distinct 
parts, the fertile part being in the form of a spike which 
arises at the junction of the petiole with the sterile lamina, on 
its adaxial side. The fertile spike is pinnately compound in 
those genera with a compound lamina and simple in 
Ophioglossum, where the lamina is simple. Its morpho- 
logical nature has been the subject of some considerable 
discussion in the past but is now generally thought to repre- 
sent two basal pinnae which have become ontogenetically 
fused, face to face (i.e. it is believed that some early ancestor 
of the group had two fertile basal pinnae, whose primordia 
became fused during subsequent evolution). Today, the only 
evidence for the double nature of the spike lies in its vascular 

supply. 

The roots are peculiar in being completely without root 
hairs, a feature which is possibly connected with their 
mycorrhizal habit. 

Growth of the stem apex is from a single apical cell, and 
its products are characteristically soft and fleshy, for they 
are without sclerenchyma. The stem of the young sporehng 
is protostelic, but soon becomes medullated. Later on, the 
stem of Botrychium becomes solenoxyUc, i.e. there are leaf 
gaps in the xylem, but not in the single external endodermis. 
Ultimately, the appearance of a sporadic internal endo- 
dermis may give rise to a rudimentary solenostele. Botrychium 
is the only genus of living ferns to show secondary cambial 
activity, and in some species it may give rise to a consider- 
able thickness of secondary wood, composed of tracheids 
and wood-rays. Rhizomes of Helminthostachys pass through 
much the same stages of stelar organization, but the largest 
specimens go one stage further and achieve true soleno- 
stely, with an internal as well as an external endodermis. 




Fig. 20 

Botrychium: a, B. lunaria; b, fertile pinnule; c, vascular supply 
to sporangia; d, prothallus of B. virginianum; e, archegonium; 
F, embryo of B. obliquum. Ophioglossum: g, O. vulgatum; h, 
portion of fertile spike; i, vascular supply to sporangia; J, 
prothallus of O. vulgatum; k, archegonium of O. pendulum; 
L, embryo of O. vulgatum 

(f, foot; 1, leaf; r, root; s, suspensor; x, stem apex) 

(a, g, after Luerssen; b, h, Bitter; c, Goebel; d, k, Campbell; 
E, Jeffrey; F, Lyon; J, l, Bruchmann) 



Ophioglossum varies considerably in its internal anatomy, 
according to species. Some possess an outer endodermis, 
but in most species it is absent, even in the young stages. The 
leaf gaps in the xylem overlap one another, giving rise to a 
network of meristeles, which form a rudimentary kind of 
dictyostele. 

The xylem is endarch in Botrychium and Ophioglossum, 
but mesarch in Helminthostachys. The earliest formed proto- 
xylem tracheids are very similar to those of the Marattiales ; 

137 



138 THE MORPHOLOGY OF PTERIDOPHYTES 

later ones are reticulate (some being ob-reticulate) but 
scalariform tracheids are absent. ^^ A pronounced feature of 
all three genera is the distinctly bordered circular pits in the 
metaxylem tracheids, but early accounts of the universal 
presence of a torus in the pit closing membrane appear to be 
incorrect. Bierhorst^^ records them only in Botrychium 
dissectum and states that even in this species they are not a 
constant feature. 

The sporangia in all three genera are 'marginal' in origin. 
In Botrychium, they are borne in two rows along the 
ultimate pinnules of the fertile spike (Fig. 20B) and each 
receives its own separate vascular supply from a vein running 
into the pinnule (Fig. 20C). In Helminthostachys, the axis of 
the fertile spike bears numerous 'sporangiophores' in several 
rows, each bearing several sporangia and a few tiny green 
lobes at the tip. The spike of Ophioglossum bears two rows 
of sporangia fused together, beyond which the axis projects 
as a sterile process (Fig. 20G). A number of vascular bundles 
run longitudinally up the middle, anastomosing occasionally 
and giving off lateral branches to the sporangia (Fig. 20I). 

Early stages of development of the sporangium are similar 
to those in the Marattiales ; a single initial cell undergoes a 
perichnal division, the inner half giving rise ultimately to 
the archesporial tissue, while the outer half goes to form 
part of the sporangium wall. Adjacent cells contribute 
further to the wall, which is very massive and several cells 
thick at maturity. A tapetum of several layers of cells is 
formed from the inner regions of the sporangium wall, 
which break down to form a continuous Plasmodium in 
which the spores develop. As in Marattiales, there are 
stomata in the sporangium wall. 

Dehiscence of the sporangium is transverse in Botrychium 
and Ophioglossum (Figs. 20B and 20H), but longitudinal in 
Helmifithostachys, and large numbers of spores are released 
(more than 2,000 in Botrychium and as many as 15,000 in 
Ophioglossum). 



PTEROPSIDA 139 

The prothallus in all three genera is mycorrhizal. Indeed, 
the presence of the appropriate fungus is essential for the 
growth of the prothallus beyond the first few cell divisions. 
In most cases the prothallus is deeply buried in the soil and 
lacks chlorophyll, but cases have been reported of super- 
ficial prothalli, in which some chlorophyll was present. 
Some have abundant rhizoids, but others are completely 
without them. 

The prothallus of Botrychium virginianum (Fig. 20D) is a 
flattened tuberous body, up to 2 cm long. Antheridia appear 
first and are deeply sunken. Large numbers of antherozoids 
are liberated from each and escape by the rupturing of a 
single opercular cell. The archegonium has a projecting neck 
several cells long, a neck canal cell with two nuclei, and a 
ventral canal cell (Fig. 20E). 

The prothallus of Ophioglossum vulgatum differs in being 
cyhndrical, and may be as much as 6 cm long (Fig. 20J). 
Frequently, there is an enlarged bulbous base, in which the 
bulk of the mycorrhizal fungus is located. (In both Figs. 
20D and 20 J, the extent of the fungus is indicated by a broken 
line.) As in Botrychium, the antheridia are sunken and pro- 
duce very large numbers of antherozoids. Unlike Botrychium, 
however, its archegonia are sunken too. In Fig. 20K, the 
archegonium is illustrated at a stage just before maturity, 
when there are visible two nuclei in the neck canal cell, but 
just before the basal cell has divided. Indeed, a ventral canal 
cell has rarely been seen, presumably because it disintegrates 
almost as soon as it is formed. 

As in the Marattiales, the first division of the zygote is in a 
plane at right angles to the archegonial axis. In Helmintho- 
stachys, the outer (epibasal) hemisphere undergoes a second 
division, so as to produce a suspensor of two cells, while 
the hypobasal hemisphere gives rise to a foot, a root and, 
later, the stem apex. The embryo is thus endoscopic, but 
during its further development its axis becomes bent round 
through two right angles, so as to allow the stem to grow 



140 THE MORPHOLOGY OF PTERIDOPHYTES 

vertically Upwards. The embryo of some species ofBotrychium 
is likewise endoscopic and has a small suspensor (Fig. 20F), 
but in others including B. lunaria, there is no suspensor and 
the embryo is exoscopic; and this is true of all species of 
Ophioglossum (Fig. 20L). In all cases there is considerable 
delay in the formation of the stem apex, and in some species 
it may be several years before the first leaf appears above 
the ground, by which time many roots may have been 
formed. These long delays suggest that the mycorrhizal 
association is an important factor in relation to the nutrition, 
not only of the prothallus, but also of the young sporophyte. 

Chromosome counts show a surprising range within the 
group, for Botrychium has a haploid number n = 45, Helmin- 
thostachys n = 46 or 47, while in Ophioglossum vulgatum 
n = 250-260 and in Ophioglossum reticulatum n = 63i + io 
fragments. 

Despite these divergent chromosome numbers, there can 
be Httle doubt that the three genera of the Ophioglossales 
are fairly closely related, nor that they represent an ancient 
and primitive group of ferns despite the lack of fossil 
representatives. The reticulate venation of Ophioglossum, 
its consohdated fertile spike and its complete lack of a 
suspensor together suggest that it has reached a more 
advanced stage of evolution than either of the other two 
genera. As in the Marattiales, it seems that the upright 
stem is the basic condition, since even in Helminthostachys 
the young plant has an erect axis. 

Regarding the relationships between the Ophioglossales 
and the Marattiales, it is not easy to decide which characters 
are significant. Of the many characters common to the two 
groups, most indicate merely that they have reached roughly 
the same stage of evolution, rather than that they are closely 
related. These may be briefly listed as i. basically erect axis, 
2. stipules at the base of the petiole, 3. absence of scleren- 
chyma, 4. sporadic endodermis, 5. massive sporangium wall, 
with stomata, the sporangia showing a tendency to fusion, 



PTEROPSIDA 141 

6. large spore output, 7. prothallus long lived, 8. massive 
antheridium, 9. suspensor present in some, absent in others. 
Characters which suggest that the two groups are only 
distantly related are the circinate vernation of the Maratti- 
ales and their superficial sori, contrasting with the absence 
of circinate vernation from the Ophioglossales and their 
marginal sporangia. 

Osmundidae 

Osmundales 
Osmundaceae Zalesskya^ ^ Thamnopteris^ ^ Osmundites"^, 

Osmunda, Todea, Leptopteris 

The modern representatives of the Osmundales occupy an 
isolated position among the ferns, intermediate in many 
respects between the Eusporangiatae and the Leptosporan- 
giatae but not necessarily, therefore, finking the two groups 
phylogenetically, for they are an extremely ancient group 
with an almost complete fossil history extending as far 
back as the Permian. Those that have survived to the 
present day can truly be described as 'living fossils'. 

All have erect axes, bearing a crown of leaves ; and the 
same is true of the fossil members, some of which had trunks 
I m or more in height. Among the earliest representatives, 
in the Permian, were several species of Zalesskya. These had 
a sofid protostele in which there were two distinct regions of 
xylem (an inner region of short tracheids and an outer one 
of elongated tracheids forming an unbroken ring). The same 
was true of Thamnopteris Schlechtendalii, but T. Kidstonii 
had a sfightly more advanced stelar anatomy, in that the 
central region was occupied by a mixed pith of tracheids 
and parenchyma. Osmundites Dunlopii from the Jurassic 
was similar to T. Kidstonii, but the contemporaneous O. 
Gibbeana showed some dissection of the xylem ring into 
about twenty separate strands.*^ Nevertheless, the stele was 
still strictly a protostele, since there was a continuous zone 



142 THE MORPHOLOGY OF PTERIDOPHYTES 

of phloem (and, presumably, endodermis) round the out- 
side. The term 'dictyoxyHc stele' can conveniently be used 
to describe this arrangement. Poor preservation does not 
allow any statement to be made about the central pith 
regions of these two forms, but in the Lower Cretaceous 
O. Kolbei there was definitely a mixed pith. The Cretaceous 
species O. skidegatensis had a pith of pure parenchyma and 
showed a further advance in having some internal phloem, 
while O. Carnieri was the most advanced of all, in being 
truly dictyostehc. This is most interesting, for it is a con- 
dition not achieved by any modern representatives of the 
group. Most of these are no further advanced in stelar 
anatomy than the Jurassic Osmundites Gibbeana. 

Of the living genera, Osmunda (fourteen species) is wide- 
spread in both hemispheres, Leptopteris (six species) is con- 
fined to Australasia and the South Sea Islands, while Todea 
is represented by the single species T. barbara, found in 
S. Africa and Australasia. (Some taxonomists include 
Leptopteris in the genus Todea.) Only one species, Osmunda 
regalis — the 'Royal fern' — is represented in the British flora. 
Its stems are massive and branch dichotomously to form 
large hummocks. Todea barbara may have a free-standing 
trunk I m or more high, and so also may Leptopteris 
hymenophylloides, while one species of Leptopteris from 
New Caledonia attains a height of 3 m. 

A transverse section of the stem of a mature Todea (Fig. 
21K) exhibits a typical dictyoxylic condition. The central 
medulla is surrounded by separate blocks of xylem, outside 
which there is phloem and a continuous endodermis. 
Occasionally, some internal phloem occurs, but no internal 
endodermis. Most species of Osmunda are similar, but O. 
cinnamomea sometimes has an internal, as well as an 
external, endodermis (Fig. 21B). The types of xylem element 
present are similar, in some respects, to those of the 
Marattiaceae,^^ and the position of the protoxylem ranges 
from endarch in Todea to nearly exarch in Osmunda. 



PTEROPSIDA 143 

The leaves, in most species, are leathery in texture, but 
those of Leptopteris hymenophylloides are comparable with 
those of the Hymenophyllaceae ('filmy ferns') and have a 
thin pellucid lamina, only two or three cells thick, from 
which stomata are completely lacking. During their develop- 
ment the leaves of all species exhibit circinate vernation and 
are covered with hairs. The base of the petiole is broad and 
winged in a manner reminiscent of the Eusporangiatae and, 
after the frond has been shed, the leaf base is persistent, 
adding considerably to the diameter and the mechanical 
strength of the stem. 

The fronds of Osmunda regalis are twice pinnate, those 
produced first in each season being sterile. These are followed 
by partially fertile fronds (Fig. 21 A), while the last to be 
produced are often completely fertile. The fertile pinnules 
are very reduced tassel-Hke structures, representing just the 
midrib. In the absence of a lamina, the sporangia cannot be 
'superficial' and are usually described as 'marginal'. In 
partially fertile fronds of O. regalis, the fertile pinnules 
occupy the distal regions, but in those of O. Claytoniana 
they occupy the middle regions. Todea bar bar a has once- 
pinnate fronds in which the fertile pinnules show scarcely 
any modification and the sporangia are superficial, being 
densely scattered over the under-surface of the lamina. 
They occupy the basal regions of partially fertile fronds 
(Fig. 21H). The fronds of Leptopteris hymenophylloides are 
large and many times pinnate, with the sporangia scattered 
sparsely along the veinlets of unmodified pinnules (Fig. 
2 1 F). In no case is there any tendency for the sporangia to 
become aggregated into sori, nor is there any sign of an 
indusium. 

The sporangium is not strictly leptosporangiate, for 
several cells play a part in its initiation and, at maturity, it 
is relatively large and massive with a stout short stalk. 
There is some variation in the shape of the archesporial cell, 
as illustrated in Figs. 21I and 21 J, for it may be tetrahedral, 



144 THE MORPHOLOGY OF PTERIDOPHYTES 

as in leptosporangiate ferns, or it may be cubical, as in the 
Eusporangiatae. The tapetum is formed from the outermost 
layers of the sporogenous tissue, unlike that of the Euspo- 
rangiatae, and there is also a layer of tabular cells, formed 
from the same regions, which becomes appressed to the 
inner side of the sporangium wall. For this reason, at 
maturity, the wall appears to be two cells thick. There is a 
primitive kind of annulus, formed by a group of thick-walled 
cells, on one side of the sporangium and a thin-walled 
stomium, along which dehiscence occurs, extends from it 
over the apex of the sporangium (Fig. 21G). Relatively large 
numbers of spores are released from each sporangium (e.g. 
about 128 in Leptopteris and more than 256 in Osmunda and 
Todea). The spores contain chlorophyll and must germinate 
rapidly if they are to do so at all. 

The prothallus (Fig. 2tC) is large, fleshy and dark green, 
resembhng a thalloid liverwort, up to 4 cm long. The 
antheridia (Fig. 21D) project from the surface, as in Lepto- 
sporangiatae, but are larger, have more wall cells and pro- 
duce a greater number of antherozoids than do most of 
them. The archegonia (Fig. 21E) are borne along the sides 
of the midrib; they have projecting necks and differ from 
those of leptosporangiate ferns only in the number of neck 
cells (six tiers, instead of the usual four). 

The embryology of the young sporophyte, too, shows 
some features which distinguish the Osmundales from the 
Leptosporangiatae. Not only is the first division of the 
zygote vertical, but so also is the second. It is the third 
division which is at right angles to the axis of the arche- 
gonium, instead of the second. Subsequent divisions are 
somewhat irregular and the embryo remains spherical for a 
relatively long time. Ultimately, however, a shoot apex, 
cotyledon, root and a large foot appear, but there is some 
irregularity in their derivation from the initial octants. 

Despite the marginal position of the sporangia in 
Osmunda, as compared with their superficial position in the 




Fig. 21 

Osmunda: a, partly fertile frond of O. regalis; b, stele of O. 
cinnamomea; c, prothallus of O. Claytoniana; d, antheridium ; 
E, archegonium; g, sporangium. Leptopteris: f, fertile pinnule 
of L. hymenophylloides. Todea: h, fertile frond of T. barbara; 
I, J, sporangial primordia (i with tetrahedral archesporial cell, 
J with cubical archesporial cell) ; k, stele 

(a, f, h, after Diels; c-e, Campbell; g, Luerssen; i, J, Bower; 
K, Seward and Ford) 

other two genera, the three genera are so similar in other 
respects that they are, without doubt, closely related, and 
this conclusion is supported by chromosome counts. The 
haploid number is n = 22 throughout. 



Filicales 

In the past, the name FiHcales was applied in the broadest 
possible sense, so as to include all the ferns but, recently, its 
use has been restricted, and it is applied just to the homo- 
sporous leptosporangiate ferns (as in Engler's Syllabus der 

145 



146 THE MORPHOLOGY OF PTERIDOPHYTES 

Pflanzenfamilien^^). However, even when thus restricted, it 
is still by far the largest group of the pteridophytes, for it 
contains almost 300 genera and about 9,000 species. Details 
of their form and anatomy would occupy many volumes 
and can only briefly be summarized here, the following 
famihes, subfamiUes and genera having been selected to 
illustrate the salient points (the classification is based on that 
of Holttum^^). 

Schizaeaceae Sehftenbergia* , Klukia*, Schizaea, Lygodium, 

Mohria, Anemia 
Gleicheniaceae Oligocarpia* , Gleichenites*, Gleichenia 
Hymenophyllaceae Hymenophyllum, Trichomanes 
Dicksoniaceae Coniopteris*, Dicksonia, Cibotium 
Matoniaceae Matonidium*, Matonia 
Dipteridaceae Clathropteris* , Dictyophyllum*, 

Camptopteris* , Matonia, Phanerosorus 
Cyatheaceae Alsophilites*, Alsophila, Hemitelia, Cyathea 
Dennstaedtiaceae 

Dennstaedtioideae Dennstaedtia, Microplegia 

Pteridoideae Pteridium, Pteris, Acrostichum (?) 

DavalUoideae Davallia 

Oleandroideae Nephrolepis 

Onocleoideae (?) Onoclea, Matteuccia 

Blechnoideae Blechnum, Woodwardia 

Asplenioideae Asplenium, Phyllitis 

Athyrioideae Athyrium 

Dryopteridoideae Dryopteris, Polystichufn 

Lomariopsidoideae Elaphoglossum 

Adiantaceae Adiantum, Cheilanthes, Pellaea, 

Ceratopteris, Anogramma 

Polypodiaceae Platy cerium, Polypodium, Stenochlaena(J) 

As might be expected in such a large group, there is a 
considerable range of form and growth habit, from tiny 
annuals to tall tree-ferns and from protosteUc forms to those 



PTEROPSIDA 



147 



with highly dissected polycycUc dictyosteles, yet all are aUke 
in the early stages of development of the sporangium. This, 
together with its stalk, arises from a single cell. The first 
division of the initial cell (Fig. 22O) is into an apical cell (i) 




Fig. 22 

Development of gametangia and sporangia as found in lepto- 
sporangiate ferns, a, typical gametophyte. b-h, stages in develop- 
ment of antheridium (diagrammatic), i, dehiscing antheridium. 
J, antherozoid of Pteridium. k-n, stages in development of 
archegonium. o-s, stages in development of sporangium of 
Polypodium 

(1, apical cell; 2, basal cell; 3, jacket cell) 

(a, k-n, o-s, after Foster and Gifford; b-h, Davie; J, Sadebeck) 

and a basal cell (2). Further divisions take place in each 
(Fig. 22P) and give rise to a primary sporogenous cell 
(shaded in Fig. 22Q) and a jacket cell (3). The former gives 
rise to a two-layered tapetum and to a number of spore 
mother cells, surrounded by a sporangium wall one cell 
thick. Further details of sporangium development differ 
according to species, for some have a long slender stalk, 
only one cell thick, while others have a short and relatively 



148 THE MORPHOLOGY OF PTERIDOPHYTES 

thick stalk; the majority have a vertical row of thick-walled 
cells, constituting the annulus, while some have an obhque 
row and others merely a group of thick-walled cells ; some 
have a high spore output, while in most species it is thirty- 
two or sixty-four. 

Most commonly the prothallus is either cordate or 
butterfly-shaped ranging in size from a few mm to i cm or 
more across. There is a midrib several cells thick, but the 
wings of the prothallus are only one cell thick. It is surface- 
living, green and photosynthetic, and there are rhizoids on the 
underside, among which antheridia and archegonia are borne ; 
the archegonia are usually concentrated near the growing 
point, or 'apical notch'. Departures from this typical form 
occur in certain famihes, e.g. some have filamentous pro- 
thalli, resembhng an algal filament, while even subterranean 
prothalli are known, but this habit is extremely rare. 

Stages in the development of the archegonium are illus- 
trated in Figs. 22K-N, the only variations being in the 
number of tiers of neck cells at maturity. The structure of the 
antheridium is also fairly constant throughout the Filicales. 
Figs. 22B-H represent the various stages in the development 
of the commonest type. The way in which successive cross 
walls bulge upwards or downwards is peculiar and is res- 
ponsible for the formation of the characteristic ring-shaped 
cells of which the mature antheridium wall is constructed.^^ 
At maturity, the cap cell is pushed off (Fig. 22I) to release 
the antherozoids (usually thirty-two in number) (Fig. 22J). 
Some famihes have a slightly more massive antheridium, 
composed of a greater number of wall cells and containing 
more antherozoids ; these are beheved to be more primitive 
than the rest. 

The embryology of the leptosporangiate ferns is Hkewise 
very constant throughout. The first cross-wall is almost 
invariably longitudinal and the second transverse. Thus, the 
zygote is divided at a very early stage into four quadrants, 
two directed towards the apical notch of the gametophyte 



PTEROPSIDA 149 

(called the inner and outer anterior quadrants) and two 
away from the notch (called the inner and outer posterior 
quadrants). The outer anterior quadrant ultimately gives 
rise to the first leaf, the inner anterior to the shoot apex, the 
outer posterior to the first root, and the inner posterior to 
the foot. This, at least, is the procedure described in 
classical studies, but more recently it has been stated that 
the fate of the four quadrants is not always so clearly 
defined. ^^ 

Statements that certain characters are primitive and others 
advanced can be made with more certainty for the Filicales 
than for any other group in the plant kingdom, because of 
the large number of fossil representatives that are known. 
Some of the famiUes had already become widespread by the 
Mesozoic, while others appeared as long ago as the Car- 
boniferous. A comparison of these with the rest of the living 
Fihcales makes it possible to draw up an extensive list of 
primitive characters for the group as a whole. The following 
list is based on that of Bower^ (as modified by Holttum^^) 
with additions by Stokey.^^ 

Rhizome — slender, creeping, dichotomous, with fronds in 
two ranks on its upper side, protosteUc, covered with 
hairs. 

Fronds — large, amply branched, dichotomous and of un- 
limited growth, the stipe (petiole) receiving a single leaf 
trace, the ultimate pinnules narrow and with a single 
vein; venation without anatomoses (i.e. *open'). 

Sort — containing few sporangia, terminating a vein. 

Sporangia — relatively large, with stout stalk, without a 
specialized annulus, developing and dehiscing simul- 
taneously to liberate a large number of spores. 

Spore germination — giving a plate rather than a filament 
of cells. 

Gametophyte — relatively large, thalloid, with a thick mid- 
rib, slow to develop. 



150 THE MORPHOLOGY OF PTERIDOPHYTES 

Antheridium — large, containing several hundred anthero- 

zoids ; wall cells more than four in number. 
Archegonium — with a relatively long neck. 

In the more advanced ferns, the dermal appendages are 
usually scales instead of hairs and, as the stem assumes an 
erect position, the leaves tend to form a crown at the apex. 
With increasing size, the stelar anatomy becomes more 
complex, the leaf-gaps overlap, and a dictyostele results. 
True vessels are known to occur in at least two genera. ^^ 
The fronds become reduced in size and may have a simple 
broad lamina with an entire margin and with anastomosing 
veins, while the stipe receives a number of leaf traces. In 
the most advanced ferns, the fronds are frequently 'jointed' 
at the base, i.e. they are shed by means of an absciss layer, a 
habit which may well be associated with Ufe outside the 
tropics, in regions where seasonal changes in chmate may 
be severe. Evolution of the sorus appears to have taken place 
in stages, the first of which involved a regular gradate 
sequence of development of the sporangia. The next resulted 
in a mixed arrangement of old and young sporangia within 
the sorus. Still more highly advanced is the condition des- 
scribed as 'acrostichoid,' where the individuahty of the sorus 
is lost and the sporangia form a 'felt' that covers the dorsal 
surface of the lamina, irrespective of the position of vein 
endings. 

The various stages in soral evolution are often held to be 
the most important indicators of relative advancement and, 
on this basis, many pteridologists subdivide the Filicales into 
SimpHces, Gradatae and Mixtae. It is important to reahze, 
of course, that these subdivisions represent levels of evolu- 
tion and not taxonomic groups. However, it is debatable 
whether one character should be weighted to this extent, for 
it is almost universally agreed among taxonomists that the 
maximum possible number of characters should be used in 
the assessment of phylogenetic status. If all the primitive 




100 



Fig. 23 

Circular phylogenetic classification of the Filicales. Families and 
subfamilies are arranged so that their radial position corres- 
ponds to their relative advancement (primitive near the centre; 
advanced near the outside). Broken lines enclose 'areas of 
affinity', indicating close relationship. The numbers represent 
successive grades of relative advancement, expressed as a per- 
centage ('the advancement index'), ranging from the most 
primitive (0%) to the most advanced (100%) 

characters listed above are taken into account, it is possible 
to calculate roughly an average 'advancement index' for 
each family or subfamily, ranging from o per cent (the most 
primitive) to lOO per cent (the most advanced). "^^ This has 

151 



152 THE MORPHOLOGY OF PTERIDOPHYTES 

been done for the families and subfamilies selected for 
detailed treatment, and they have been arranged (Fig. 23) on 
a circular scheme, according to their advancement index. 
The most primitive families are near the centre and the most 
advanced are near the outside. The broken hnes, enclosing 
'areas of affinity', indicate which groups are most closely 
related to each other (in the main, the views expressed here 
accord most closely with those of Holttum*^' ^^). Such a 
scheme may be thought of as a view, looking down from 
above, of the 'tree of evolution' of the Filicales, and while 
it may not be acceptable to all taxonomists, it does avoid the 
error, which is common to most phylogenetic classifications, 
of suggesting that one modern family has evolved from 
another modern family. ^^ 

The two most primitive famihes are the Schizaeaceae and 
the Gleicheniaceae, and they are also the oldest, being repre- 
sented in Carboniferous deposits by Senftenbergia and 
Oligocarpia respectively. Both are represented in the Meso- 
zoic, too (viz. Klukia and Gleichenites). 

Schizaeaceae 

The Schizaeaceae are represented today by four genera and 
about 160 species, most of which are tropical or subtropical 
in distribution. In all of them, the sporangia are borne 
singly instead of in sori ('monosporangial sori') and they 
show the most primitive type of dehiscence mechanism 
known in the FiUcales. In all, the annulus consists merely of 
a terminal group of thick-walled cells (Figs. 24A-D) and 
dehiscence is longitudinal. The stalk of the sporangium is 
short and thick and the spore output from each is 128 or 256. 
The sporangia arise simultaneously, on the margin of the 
frond, and are unprotected, except by the inrolUng of the 
margin, or marginal flaps, of the pinnule. Lygodium is one of 
the few modern genera of ferns to have fronds of unlimited 
growth, forming twining structures 30 m or more in length. 
Unlimited growth is a feature which, in most plants, is taken 



PTEROPSIDA 



153 



to distinguish stems from leaves. When it occurs in fronds, 
as in this case, it is, therefore, taken as evidence that they 
have evolved from stem structures (or are still in the process 
of doing so). Further evidence that the frond of Lygodium 




Fig. 24 

Sporangia of Filicales: a, Anemia; b, Schizaea; c, Lygodium; 
D, Mohria; E, F, G, Gleichenia; H, i, Matonia; J, k, l, Hymeno- 
phyllum; M, N, Cibotium; o, p, Hemitelia; Q, R, s, Dipteris; 
T, u, Adiantum 

(a-d, after Prantl; e-s, Bower; t, u, Miiller) 

is very primitive is provided by the structure of the leaf 
trace, which shows only slight departures from radial 
symmetry. The other three genera have leaf traces which are 
clearly dorsiventral and 'gutter-shaped'. Their stem struc- 
tures, too, are more advanced for, whereas Lygodium has a 
creeping protostelic rhizome, Schizaea has an oblique 
rhizome with a medullated protostele, Anemia has a creep- 
ing or oblique rhizome which is either solenostelic or dictyo- 
steHc, while Mohria is dictyosteHc. It is interesting to note, 



154 THE MORPHOLOGY OF PTERIDOPHYTES 

also, that Mohria is the most advanced in its dermal 
appendages, for they are glandular scales, whereas those of 
the other three genera are hairs. In Anemia, only the two 
lowermost pinnae are fertile. 

The prothalli are flat thalloid structures, except in Schizaea, 
where they are filamentous, with occasional mycorrhizal 
cells and with the gametangia at the tips of short lateral 
filaments. Bower remarked that these filamentous prothalli 
are 'the simplest prothalli known among the Pteridophyta. 
They suggest a primitive state, and provoke comparison 
with green Algae'. However, their simplicity is now regarded 
as the result of evolutionary speciaHzation, instead of repre- 
senting a primitive state. 

Of the four genera, Lygodium has the most complex 
antheridial wall and the highest output of antherozoids 
(156). 

Gleicheniaceae 

This family is represented by about 130 species belonging, 
mostly, to the one genus, Gleichenia (some taxonomists 
prefer to split the genus into four). A number of rather 
different types of leaf morphology occur, two of which are 
illustrated in Figs. 25C and 25D, but in all of them the growth 
of the main rachis is arrested, until a pair of primary laterals 
has formed. In some species, these are of limited growth 
(Fig. 25C), but in others they too may terminate in dormant 
buds, so producing a variety of patterns, some looking 
superficially like a series of regular dichotomies (although, 
in fact, they are psuedo-dichotomies, because of the dormant 
apical bud in each angle). In others, there is a zig-zag 
arrangement of branches (Fig. 25D). As in the Schizaeaceae, 
therefore, the fronds are of indefinite growth, and some 
attain a length of 7 m or more. They arise from a creeping 
dichotomous rhizome which in most species is protosteHc. 
A few, however, achieve a solenostelic condition, e.g. 
G. pectinata, a relatively advanced condition which is associ- 



PTEROPSIDA 155 

ated with a larger number of sporangia in the sorus than is 
usual in the genus. Yet, in this species, the dermal append- 
ages are hairs, whereas scales are commonly present in 
others. Divergent facts such as these serve to emphasize the 
point that the evolution of different characters does not 
necessarily keep step, the result being that most organisms 
show a combination of advanced and primitive characters. 
This is why it is unwise to focus attention unduly on one 
character, when attempting to assess the relative advance- 
ment of taxonomic groups. 

The sporangia, in strong contrast to those of the Schizae- 
aceae, are borne superficially on the adaxial side of the 
frond. They develop simultaneously and are arranged in 
sori containing, often, only a single ring of sporangia, seated 
either at a vein ending or, more usually, over the middle of a 
vein. There is no indusium at all covering the sorus, whose 
only protection is a covering of hairs or scales. Each 
sporangium is pear-shaped (Figs. 24E-G), has a stout stalk, 
and dehisces by means of an apical slit. Dehiscence is 
brought about by the contraction of the thickened cells of 
the annulus, which runs obliquely round the sporangium 
wall. Large numbers of sporangia are liberated from each, 
ranging from 128 to more than 1,000. 

The gametophyte is primitive, in that it is large, massive 
and slow growing. When old, it becomes much fluted and 
develops an endophytic mycorrhizal association. The anther- 
idia are larger than in any other leptosporangiate fern and 
resemble those of the Osmundales. Those of G. laevigata are 
as much as ioo/a in diameter and contain several hundred 
antherozoids. 

Hymenophyllaceae 

This group is commonly referred to as 'the filmy ferns', 
because of their delicate fronds, the lamina of which is 
usually only one cell thick. There are some 300 species of 
Hymenophyllum, of which two occur in the British Isles, and 




Fig. 25 

Leaf form: a, Phanerosorus sarmentosiis ; b, Matonia pectinata; 
c, Gleichenia longissima; d, G. linearis, var alternans. Sori: E, 
Matonia pectinata ; F, Trichomanes alatum; g, Cibotium Barometz; 



PTEROPSIDA 157 

350 of Trichomanes, of which there is one in the British Isles. 
Because of their deUcate nature, almost all of them are 
confined to moist habitats, and most of them are restricted 
to the tropics, where they commonly grow as epiphytes. 
The British species H. tunbrigense may be seen growing on 
rocks constantly wetted by the spray from waterfalls. 

Most filmy ferns have a thin wiry creeping, protostelic, 
rhizome, from which the fronds arise in two rows. In one 
species, the stele of the rhizome is reduced to a single 
tracheid, while in another there is said to be no xylem at 
all. Some species are completely without roots. The leaf 
trace is a single strand, which at the base of the stipe 
shows marked similarity to the stem stele but, higher up the 
stipe, broadens out into a gutter-shaped strand. The frond 
is usually much branched, each narrow segment having a 
single vein, but various degrees of 'webbing' occur and, in 
one species, Cardiomanes reniforme ( = Trichomanes reni- 
forme), there is a single expanded lamina. Nevertheless, the 
venation is open in all species. 

The sori are marginal, and most species are strictly 
gradate. The vein leading to the sorus continues into a 
columnar receptacle which, in Trichomanes, can grow by 
means of an intercalary basal meristem until it forms a 
slender bristle. The receptacle of Hymenophyllum has more 
limited powers of growth or may lack them altogether. In 

H, Cyathea Dregii; i, Dennstaedtia cicutaria; J, Micro lepia 
Speliincae; k, Matteuccia struthiopteris {=Struthiopteris germa- 
nicd); l, Dryopteris filix-mas; m, Polystichum lobatum; N, 
Nephrolepis davallioides ; o, Pteris tripartita; p, Pteris cretica; 
Q, Pteridiiim aqidlinum; R, Athyrium filix-femina; s, Adiantum 
Parishii; T, Lomaria spicant; u, Blechnum occidentale; v. 
Phyllitis scolopendrium : w, Asplenium lanceolatum 

(1, outer indusium; 2, inner indusium) 

(a, b, j, u, V, after Diels; c, d, o, Holttum; f, Eames; g, h, s, 
Hooker; i, Baker; k, l, m, w, Luerssen; n, r, Mettenius; p, q, 
T, Bower) 



158 THE MORPHOLOGY OF PTERIDOPHYTES 

such species, the sporangia are produced simultaneously, 
but, where the receptacle can grow, new sporangia arise in 
basipetal sequence. Surrounding the sorus is a cup-shaped 
indusium in Trichomanes (Fig. 25F, where the broken Hne 
indicates where the indusium was cut away to show the base 
of the receptacle) and a two-lipped indusium in Hymeno- 
phyllum. 

The sporangium has a relatively thin stalk and an oblique 
annulus, which brings about dehiscence along a lateral line 
(Figs. 24J-L), by a process of slow opening, followed by 
rapid closure as a gas phase suddenly appears in the cells 
of the annulus. This mechanism is found throughout the 
more highly evolved members of the Fihcales, and results in 
the forcible ejection of the spores. The spore output varies 
from 128 or 256 in Hymenophyllum to as low as thirty-two 
in some species of Trichomanes. 

The prothallus of Hymenophyllum is a strap-shaped 
thallus, often only one cell thick, but, by contrast, the few 
species of Trichomanes whose prothalH have been studied 
have a filamentous structure which, like that of Schizaea, is 
mycorrhizal. 

Dicksoniaceae 

The first recorded occurrence of a fossil member of the 
Dicksoniaceae is of Coniopteris, from Jurassic rocks of 
Yorkshire. Like modern members of the group, it had highly 
compound fronds with marginal sori, protected by two flaps 
(the upper and lower indusia). In the modern genus 
Cibotium, the fronds are borne on stout creeping stems or on 
low massive trunks, while some species of Dicksonia are tall 
tree-ferns (e.g. D. antarctica), with a crown of leaves at the 
summit of a tall trunk. All are characterized by a profuse 
hairy covering over the stem and the base of the stipe, the 
hairs being as much as 2 cm long in Cibotium barometz. 

The stems are solenosteUc or (in species with erect axes) 
dictyostelic, and the stele is deeply convoluted around a 



PTEROPSIDA 159 

large central pith region. There is a single gutter-shaped 
strand entering the base of the stipe, but this soon breaks up 
into numerous small bundles. 

The sporangia are truly marginal in origin and arise in 
strictly gradate sequence within a purse-hke box, formed by 
the two indusia (Fig. 25G). They are long-stalked and have 
an obhque annulus (Figs. 24M and 24N) which, in some 
species, is very nearly vertical. The typical spore output per 
sporangium is sixty-four. 

Matoniaceae 

This is a most interesting family, containing the two genera 
Phanerosorus, from Sarawak and New Guinea, and Matonia, 
from Malaya, Borneo and New Guinea. In spite of its 
rarity at the present day, the family had many fossil repre- 
sentatives in the Triassic. So characteristic is the method of 
branching of the frond (Fig. 25B) that there can be little 
doubt that the fossil Matonidium is correctly placed in this 
family. After an initial dichotomy, each half of the frond 
undergoes a regular series of unequal catadromic dicho- 
tomies (i.e. each takes the main growing point further from 
the median plane). Each pinna is pinnatifid and there are 
anastomoses in the veinlets, particularly in the neighbour- 
hood of the sori. Phanerosorus (Fig. 25A) has a frond of 
indefinite growth which is long and slender and bears 
dormant buds at the tips of some of its branches. 

The stem of Matonia is creeping and hairy, and has a very 
characteristic polycycHc stelar structure, with two co-axial 
cylinders surrounding a central sohd stele. From these, a 
single gutter-shaped leaf trace is formed, both cyhnders 
playing a part in its origin. 

The sori are superficial and consist of a small number of 
sporangia arranged in a ring round the receptacle, which 
continues into the stalk of an umbrella-shaped indusium 
(Fig. 25E represents a vertical section through a young 
sorus). There is an oblique convoluted annulus round the 



l60 THE MORPHOLOGY OF PTERIDOPHYTES 

Sporangium, dehiscence being lateral, although there is no 
special stomium of thin-walled cells (Figs. 24H and 24I). 
The spore output is sixty-four. 

Dipteridaceae 

This family is represented at the present day by some 
eight species of the single genus Dipteris, restricted to the 
Indo-Malayan region, but in Triassic times there were at 
least three genera, Clathropteris, Dictyophyllum and Camp- 
topteris. Again, the architecture of the leaf is quite character- 
istic, and there can be little doubt as to the correct taxonomic 
placing of these fossil forms. After an initial dichotomy, the 
frond shows successive unequal dichotomies in an anadromic 
direction (i.e. towards the median plane). This pattern is 
represented in present-day species, in the venation of the 
two halves of the frond. However, while the primary veins 
are dichotomous, the smaller ones form a reticulum of a 
highly advanced type, with blind-ending veinlets, as in the 
leaves of many flowering plants. 

The fronds arise at distant intervals along a creeping hairy 
rhizome, whose vascular structure is a simple solenostele. 
While some species have only a single leaf trace, others have 
two entering the base of the stipe. 

The sorus is superficial, completely without an indusium, 
and the sporangia are interspersed with glandular hairs. In 
Dipteris Lobbiana the sporangia arise simultaneously, but in 
D. conjugata they are mixed. Thus, the single genus cuts 
right across the division of the ferns into Simplices, 
Gradatae and Mixtae. 

The sporangia have relatively thin stalks (only four cells 
thick) the annulus is obUque (Fig. 24Q-S), and dehiscence 
is lateral. The spore output is sixty-four. 

Cyatheaceae 

This is the family to which most of the tree-ferns of the world 
belong. Indeed, at one time, all tree-ferns were placed in it, 



PTEROPSIDA l6l 

but Dicksonia must clearly be removed on account of its 
marginal sori, for the Cyatheaceae, as now constituted, have 
superficial sori. The earUest known fossil representative of 
the group is ^/^o;?/;////^^ from the Jurassic. Bower^ recognized 
three Uving genera within the family: Alsophila with about 
300 species, Hemitelia with about 100, and Cyathea with 
about 300. 

Although the largest may attain a height of 25 m, some 
species are comparatively low-growing. Much of the dia- 
meter of the trunk is composed of matted adventitious roots 
and persistent leaf bases, while the stem within is relatively 
small. Nevertheless, its stelar anatomy is highly complex for, 
in addition to a convoluted dictyostele, there are abundant 
medullary strands, and sometimes cortical strands too. 
Broad chaffy scales form a dense covering over the stem 
apex and the base of the frond. 

The stipe receives a number of separate leaf-traces from 
the lower margin of the associated leaf gap. While the fronds 
of most species are several times pinnate, those of Cyathea 
sinuata are simple. The venation is open in the majority of 
species, except for very occasional vein fusions. 

The three genera recognized by Bower are distinguished 
by the character of the indusium but, otherwise, the sori 
are very similar in their gradate development. In Alsophila 
there is no indusium at all, in Hemitelia there is a large 
scale at one side of the receptacle, and in Cyathea (Fig. 25H) 
it extends all round the receptacle to form a cup which com- 
pletely covers the globose sorus when young, but which 
becomes torn as the sporangia develop and push through it. 
Holttum,^^ however, regards this distinction between the 
three genera as artificial, and prefers to merge them into the 
one genus Cyathea. Furthermore, he has recently changed 
his opinion as to the affinities of the family, for he now 
draws attention to the close similarity between the scale- 
like indusium of some species and the lower indusium of 
Dicksonia.'^^^ * 



l62 THE MORPHOLOGY OF PTERIDOPHYTES 

The sporangium is relatively small, with a four-rowed 
stalk, an oblique annulus (Figs. 24O and 24P), and a fairly 
well marked lateral stomium. The spore output ranges from 
sixty-four to sixteen, and even eight in some species. 

We now come to the large assemblage of ferns whose sori 
show the mixed condition and which Bower grouped to- 
gether in the one big artificial family, the Polypodiaceae. 
Some, he believed, had affinities with the Dicksoniaceae, 
some with the Cyatheaecae and some with the Osmundaceae, 
yet all had achieved the same advanced type of sporangial 
structure, with a thin stalk, a vertical incomplete annulus, 
and lateral dehiscence. Figs. 24T and 24U are two views of 
the sporangium of Adiantum, which demonstrate the small 
number of cells constituting the capsule, and the way in 
which the stalk is composed of just one row of cells, in the 
most highly evolved types. 

In 1949 Holttum^^ suggested a more nearly natural 
classification of these ferns, by creating a new family, the 
Dennstaedtiaceae, within which he grouped a number of 
subfamihes which, he believes, have affinities with the 
Dicksoniaceae. In this new scheme of classification, the 
Polypodiaceae constitute a very restricted family, having 
affinities with the Matoniaceae, the Dipteridaceae being 
absorbed into it. Within the Dennstaedtiaceae, so many 
evolutionary processes have taken place that the group is 
hard to define; indeed, it would almost seem that the sub- 
families warrant elevation to family status. 

Dennstaedtioideae 

This is the most primitive of the subfamihes of the 
Dennstaedtiaceae, for some species still retain the gradate 
arrangement of sporangia in the sorus. Most have creeping 
rhizomes with solenosteles. The sorus of Dennstaedtia (Fig. 
25I) is very similar indeed to that of Dicksonia in having two 
indusia. In Microlepia, however, the upper indusium is 



PTEROPSIDA 163 

greatly expanded (Fig. 25J), so that, in spite of its marginal 
origin, the sorus appears to be superficial at maturity. This 
represents an early stage in the evolutionary process which 
Bower called the *Phyletic Slide', whereby the sorus ulti- 
mately has a superficial origin despite is marginal ancestry. 

Davallioideae 

Davallia, likewise, has a superficial sorus at maturity, 
covered by a funnel-shaped indusium, but which, neverthe- 
less, is marginal in origin. The stem is creeping, with a peculiar 
type of dissected solenostele, and is clothed with scales. 

Oleandroideae 

Nephrolepis has upright, dictyostelic stems with long run- 
ners, by means of which vegetative reproduction occurs, for 
the tips of the runners are capable of rooting and turning 
into normal erect stems. Within the genus, there is a wide 
range of soral form. A'^. davallioides (Fig. 25N) is very 
similar to Microlepia, in that the upper indusium is scarcely 
larger than the lower. In A'^. acuta, the sorus is superficial, 
not only at maturity, but also in origin. A'^. dicksonioides 
shows a different evolutionary trend, in that adjacent sori 
are sometimes fused, and this trend has proceeded so far in 
A^. acutifolia that the margin of the pinna has a sorus run- 
ning continuously along it, between two linear indusia. 

Pteridoideae 

It is generally accepted that the sorus of Pteridium evolved 
in a similar way to that of Nephrolepis acutifolia, for it, too, 
is continuous along the margin of the pinnule (Fig. 25Q) 
and is protected by two indusia. The upper indusium (i) is 
relatively thick, but the lower one (2) is thin and papery. 
Pteridium is one of the most successful ferns in its ability to 
compete with flowering plants and this may, to some extent, 
be due to the great depth at which its rhizomes spread be- 
neath the surface of the soil. Its stele is a dicycHc perforated 



164 THE MORPHOLOGY OF PTERIDOPHYTES 

solenostele. Pteris also has a continuous sorus near the 
margin of the lamina (i.e. the sorus is superficial in origin) 
and the margin becomes inrolled to protect it (Fig. 25O). 
In some species, e.g. Pteris cretica (Fig. 25P), the soral 
region is somewhat expanded and indicates the way in 
which the acrostichoid condition might have evolved. Indeed, 
Bower suggested a close relationship between Pteris and 
Acrostichum. Chromosome counts support this view, but 
they also suggest that Pteris is wrongly classed with 
Pteridium. The haploid numbers are, for Pteridium fifty- 
two, for diflferent species of Pteris twenty-nine and 120 and 
for Acrostichum thirty. The fact that in most of the Adianta- 
ceae n = 29 or 30 suggests a possible affinity between Pteris 
(and Acrostichum) and this family. It should be noted that 
Lygodium, too, has a haploid number n = 29 or 30. Holttum 
believes that another acrostichoid genus Stenochlaena is 
closely related to Acrostichum, but a chromosome number 
lying somewhere between seventy and eighty casts some 
doubt on this. Bower placed it near Blechnum but, for the 
time being, Stenochlaena should perhaps remain unplaced. 

Onocleoideae 

Holttum leaves this subfamily unplaced in his classification, 
while Bower thought that it shows some affinities with the 
Cyatheaceae and with the Blechnoideae. It contains two 
genera, Matteuccia (two species) and Onoclea (monotypic). 
Both are markedly dimorphic, with specially modified 
fertile fronds. The fertile pinnae are narrow and the margins 
are tightly inrolled so that protection of the sorus is derived 
more from them than from the indusium, which is thin and 
papery (Fig. 25K). Both are dictyostelic and covered with 
scales. Matteuccia has open venation and Onoclea reticulate. 

Dryopteridoideae 

The ferns in this subfamily have a short stout stem which is 
more or less erect, covered with scales and dictyostelic. 



PTEROPSIDA 165 

The stipe receives numerous leaf traces, and the venation is 
open. The sori are superficial on the veins, or at vein endings, 
and are covered by an indusium which in Dryopteris is 
reniform (Fig. 25L) and in Polystichum is peltate (Fig. 25M). 
Of these the reniform type is probably the more primitive, 
for it is not far removed from the condition figured for 
Nephrolepis (Fig. 25N). From this type, it is easy to 
imagine the evolution of the radially symmetrical indusium 
of Polystichum, by the extension of the 'shoulders' round the 
point of attachment, followed by a 'fusion' to form a disc, 
with a central point of attachment. 

Athyrioideae 

Some species of Athyrium have indusia that are identical in 
shape with those of Dryopteris, but most have two types on 
the same frond, as does the British A.filix-femina (Fig. 25R). 
Here, there are some sori with reniform indusia and some in 
which the indusium is extended along the lateral veins. The 
vascular supply to the stipe of the frond consists of two leaf 
traces, which unite into a single gutter-shaped strand higher up. 

Lomariopsidoideae 

All the members of this subfamily are acrostichoid. There 
has been much discussion as to their affinities, but they 
probably fie with the Davallioideae, for the stele of Elapho- 
glossum is very similar to that of Davallia, in having two 
large meristeles connected into a cyfinder by a network of 
smaller bundles. 

Asplenioideae 

This subfamily, too, is befieved by Holttum to have affinities 
with the Davalfioideae. The sorus of Asplenium (Fig. 25W) 
is extended along the lateral veins and is protected by an 
indusium which is usually acroscopic (i.e. its free margin is 
directed towards the apex of the pinna). In this, it resembles 
most of the sori of Athyrium. However, the vascular supply 



l66 THE MORPHOLOGY OF PTERIDOPHYTES 

to the stipe is different from that in Athyrium, for the two 
bundles which enter it fuse into a single four-armed strand, 
instead of into a gutter-shaped strand. The same is true of 
Phyllitis. That Asplenium and Athyrium are closely related 
seems fairly certain, since they have the same basic chromo- 
some number, n = 36, and hybrids between them are known 
to occur. In Phyllitis the sori occur in pairs, facing each 
other, along the lateral veins (Fig. 25V), one acroscopic 
and the other basiscopic. 

Blechnoideae 

Blechnum punctulatum forms a possible intermediate be- 
tween Phyllitis and the more typical species of Blechnum, 
for on one and the same frond both types of sorus may 
occur, some in pairs facing each other and some showing 
various degrees of fusion along a commissural vein. Wood- 
wardia has a series of box-Hke sori, on either side of the mid- 
rib, whose indusia are Uke hinged Hds. The typical Blechnum 
sorus is a continuous one, as if the adjacent sori of a Wood- 
wardia had become fused together, with the indusium facing 
the midrib of the pinna (Fig. 25U). Each has beneath it a 
commissural vein, which is visible in Fig. 25T, where part of 
the two sori have been removed to expose it. The British 
species, here figured, shows a considerable reduction of the 
fertile lamina, and this reduction process has gone much 
further in other members of the subgenus Lomaria, where the 
lamina is almost completely lacking. Such species are 
markedly dimorphic, for the sterile fronds have a normal 
unreduced lamina. The genus shows a wide range of habit, 
for some species are creeping, some are cHmbing, while 
several have erect trunks, like small tree ferns. 

Adiantaceae 

This is a very diverse family, some members of which show 
marked similarities with Mohria (Schizaeaceae). Their sori 
are without indusia and occur along the veins or else form 



PTEROPSIDA 167 

'fusion sori' near the margin, much as in Pteris. Adiantum 
has the sporangia restricted to the under side of special 
reflexed marginal flaps of the lamina (Fig. 25S). The 
majority of the members of the family inhabit fairly dry 
regions and some are markedly xeromorphic, e.g. Cheilanthes 
and Pellaea. However, at the other extreme, Ceratopteris is 
a floating, or rooted, aquatic plant, now widespread in 
tropical countries, where it chokes up canals and slow moving 
rivers. Anogramma leptophylla is interesting, in having a 
subterranean perennial prothallus, from which arise deUcate 
annual sporophytes. 

Polypodiaceae 

Within this family are placed a number of genera of ferns, 
all of which completely lack any kind of indusium. There are 
about 1,000 species in the family, almost all tropical in 
distribution (but note that Polypodium vulgare occurs in the 
British flora), and most are epiphytic. Many have highly 
complex anastomosing venation and some are acrostichoid, 
e.g. Platycerium. This genus is markedly dimorphic, with 
'nest leaves' appressed to the tree trunk on which it is grow- 
ing, while the fertile fronds are quite diff'erent in shape and 
give rise to the name 'Stag's horn fern'. 

It will be clear, from this brief survey of the Filicales, that 
there is much scope for disagreement among pteridologists 
as to the relationships and detailed phylogeny of the group, 
and that much more research is necessary before final con- 
clusions can be reached. The areas of affinity indicated in 
Fig. 23 must, therefore, be regarded as only tentative. On 
the evidence so far available, it would seem that the group 
might well be diphyletic, with two evolutionary starting 
points, one with marginal and the other with superficial 
sori. Furthermore, it seems clear that even among those with 
marginal origins there has been a trend towards the super- 
ficial condition. Should the Filicales prove to have been 



l68 THE MORPHOLOGY OF PTERIDOPHYTES 

monophyletic, however, then it is most probable that the 
ancestral type had marginal rather than superficial sori, and 
that the early Superficiales underwent a 'phyletic sUde' early 
in their evolution, while the Marginales are proceeding more 
slowly in the same direction. 

'Water Ferns' 

There are two interesting groups of leptosporangiate ferns 
which, at one time, were classified together as the Hydro- 
pterideae. Features which they show in common are hetero- 
spory and a hydrophilous habit, but in other respects they 
are so different as to warrant a much wider separation, 
from each other and from the rest of the ferns. Accordingly, 
their taxonomic status has been elevated to the Marsileales 
and the Salviniales respectively. 

Marsileales 

Pilulariaceae Pilularia 
Marsileaceae Marsilea, RegneUidium 

All the members of the Marsileales have creeping rhizomes, 
bearing erect leaves at intervals, on alternate sides. The only 
member of the group represented in the British flora is 
Pilularia globuUf era ('Pillwort'). Like all species oi Pilularia, 
its leaves are completely v/ithout any lamina (Fig. 26A). The 
leaves of the monotypic Brazihan genus RegneUidium have 
two reniform leaflets. Marsilea occurs in temperate and 
tropical regions, many of its sixty-five species occurring in 
Austraha. Its leaves have four leaflets and somewhat 
resemble a 'four-leaved clover' (Fig. 26B). All have soleno- 
stelic rhizomes but, in Pilularia, the vascular structure is 
much reduced, and the internal endodermis may be missing. 
The sporangia, in all three genera, are borne in hard bean- 
hke sporocarps, attached either to the petiole, near its base, 
or in its axil, either stalked or sessile. The morphological 
nature of these sporocarps has been the subject of much dis- 




Fig. 26 

Piliilaria: A, habit of P. globidifera. Marsilea: b, habit of M. 
quadrifolia; c, vertical section of sporocarp; d, horizontal 
section of sporocarp; e, dehiscing sporocarp; f, male game- 
tophyte; g, female gametophyte of M. vestita; h, embryo within 
female gametophyte 

(f, foot ; 1, leaf; r, root ; x, stem apex) 

(b, after Meunier; c, d, Eames — much simplified; E, Eames; f. 
Sharp; g, Campbell; h, Sachs) 



cussion, but it can most conveniently be regarded as a 
tightly folded pinna (Hke a clenched fist) enclosing a number 
of elongated sori, each covered by a membranous indusium. 
Figs. 26C and 26D represent, very diagrammatically, the 
structure of the sporocarp of Marsilea as seen in vertical and 
horizontal sections, respectively (for clarity, the number of 
sori has been reduced to two rows of five). Each receptacle 
bears microsporangia laterally and megasporangia termin- 
ally, and receives vascular bundles from a number of strands 

169 



lyO THE MORPHOLOGY OF PTERIDOPHYTES 

running down in the wall of the sporocarp. Arching over the 
top, is a gelatinous structure sometimes called a 'sporo- 
phore' (cross-hatched in the figures) which swells up at 
maturity and drags the paired sori from the sporocarp, as it 
dehisces (Fig. 26E). 

The sporocarp of Pilularia is similar in construction, 
except that there are only four sori. 

The sporangia are typically leptosporangiate in origin, 
the sporangium wall is very thin, and there is a tapetum of 
two or three layers of cells. The microsporangia contain 
thirty-two or sixty-four microspores but, in the mega- 
sporangia, all but one of the potential spores degenerate. 
On dehiscence of the sporocarp, the delicate sporangium 
wall rapidly decays and the spores begin to germinate almost 
at once. 

The male gametophyte (Fig. 26F) is extremely simple, as 
in most heterosporous plants, consisting of nine cells only. 
There is a single small prothaUial cell, and six wall cells 
surround two spermatogenous cells (cross-hatched in the 
figure) that give rise to sixteen antherozoids each. 

The first cross-wall in the germinating megaspore is ex- 
centrically placed and cuts off a small apical cell, whose 
further divisions give rise to a single archegonium (Fig. 26G) 
with a short neck of two tiers of cells, one neck canal cell, 
a ventral canal cell and a large egg cell. 

The first division of the zygote is longitudinal, and the 
second transverse, giving four quadrants (Fig. 26H) of which 
the outer two develop into the first leaf (1) and the first 
root (r), while the inner two develop into the stem apex (x) 
and the foot (f ). Meanwhile, the venter of the archegonium 
grows and keeps pace, for a time, with the enlarging embryo 
so as to form a sheath round it, from the underside of 
which a few rhizoids may be produced. The first few leaves 
in Marsilea are without a lamina and, therefore, closely 
resemble the leaves of Pilularia. 

It is often claimed that this interesting group of ferns 



PTEROPSIDA 171 

represents an evolutionary offshoot from an ancient schizae- 
aceous stock. Arguments for this are based on the leaf form, 
the type of hairs, the form of the sorus and the vestigial 
annulus round the apex of the sporangium in Pilularia,^^ 
but the evidence is not very convincing and, in the absence 
of early fossil representatives, the group must be regarded in 
the meantime as an isolated one. 

Salviniales 

Salviniaceae Salvinia 
Azollaceae Azolla 

Whereas most of the members of the Marsileales are rooted 
in the soil, either in or near water, all the members of the 
Salviniales are actually floating. Azolla has pendulous roots, 
but Salvinia is completely without them. Like the Marsile- 
ales, their sporangia are borne in sporocarps. However, the 
morphological nature of the sporocarps is quite different, 
for each sporocarp represents a single sorus whose indusium 
forms the sporocarp wall. 

The only member of the group to be represented in the 
British flora is Azolla filiculoides, described as recently 
naturahzed from N. America. However, it was a native British 
plant in Interglacial times.^^ It has an abundantly branching 
rhizome, with a minute medullated protostele, and with 
crowded overlapping leaves about i mm long (Fig. 27A). 
These have two lobes, within the upper of which is a cavity 
containing the blue-green alga Anabaena azollae. 

Sporocarps arise on the first leaf of a lateral branch and 
are usually of two kinds— large ones containing many 
microsporangia and small ones containing a single mega- 
sporangium, although sporocarps with both types of spor- 
angium are sometimes present. The early stages of develop- 
ment are similar in both types of sporocarp, for there is an 
elongated receptacle on which numerous sporangial initials 
arise. However, during development, the microsporangial 



172 THE MORPHOLOGY OF PTERIDOPHYTES 

initials abort in the one case (Fig. 27B) and the mega- 
sporangial initials abort in the other. In both types of 
sporangium there is an abundance of mucilaginous 'peri- 
plasmodium', which becomes organized into 'massulae'. In 
the megasporangium there are four such massulae, in one 
of which the single megaspore is buried. Fig. 27C illustrates 
the dehiscence of a megasporangium, the apex of which is 
cast adrift as a cap over the four massulae. The megaspore 
then germinates to produce a cap of prothallial tissue, 
within which several archegonia develop (Fig. 27D). 

When the microsporangium dehisces a variable number of 
spherical frothy massulae are liberated, each with several 
microspores near the periphery. Each bears a large number 
of peculiar anchor-hke 'glochidia' (Fig. 27E). These become 
entangled with the massulae surrounding a megaspore and, 
together, they sink to the bottom, where the microspores 
germinate, without being released from the massulae. The 
male gametophyte (Fig. 27F) has a single antheridium from 
which eight antherozoids are liberated. 

The cleavage of the zygote is typical of the leptosporangi- 
ate ferns, and as soon as the first leaf appears the sporehng 
rises, carrying the massulae etc. once more to the surface. 

There are about twelve species of Salvinia, several of 
which occur in Africa. Its horizontal floating stems, up to 
10 cm long, have a much reduced vascular anatomy and 
bear leaves in whorls of three (Fig. 27G), two floating and 
one submerged. Whereas the floating leaves are entire and 
covered with pecuHar unwettable hairs, the submerged 
leaves are finely divided into linear segments that bear a 
striking resemblance to roots (Fig. 27H). However, it is 
doubtful whether they perform the functions of roots. 
Growth is rapid and fragmentation occurs easily, with the 
result that ponds and lakes in tropical regions may rapidly 
become covered and canals choked. 

The first few sporocarps to be formed in each cluster 
contain megasporangia, up to twenty-five in each, and the 



PTEROPSIDA 



173 



later ones microsporangia, in large numbers, on branched 
stalks (Fig. 27I). All except one of the potential megaspores 
in each megasporangium abort, and the functional mega- 
spore becomes surrounded by a thick perispore (Fig. 27J), 
which later becomes cellular and comes to look, super- 
ficially, like the pollen chamber of a gymnosperm seed. 




Fig. 27 

Azolla: a, portion of plant of A. filiculoides; b, sporocarp with 
young megasporangium; c, megasporangium with massulae; 
D, the same with female prothallus; e, massula from micros- 
porangium; f, male prothallus. Salvinia: g, portion of plant of 
S. natans; h, node with sporocarps; i, sporocarps with mega- 
sporangia and microsporangia; J, megasporangium; k, micro- 
sporangium; L, male prothallus; m, archegonium; n, female 
prothallus with young sporeling attached 

(1, column; 2, leaf; 3, root) 

(b, after Pfeififer; c, e, Bernard; d, Campbell; f, l, Belajeff; 
G, H, Bischoff; i, Luerssen; J, Weymar; k, h, Yasui; n, Lasser) 



174 THE MORPHOLOGY OF PTERIDOPHYTES 

Within the microsporangium, the sixty-four microspores 
come to He at the periphery of a single frothy massula (Fig. 
27K). They remain within the sporangium throughout and, 
as they germinate, the male prothalli project all round. Each 
male prethallus contains two antheridia (Fig. 27L) pro- 
ducing a total of eight antherozoids. 

The megaspore, too, remains throughout within the 
sporangium, after it has become detached. The female pro- 
thallus protrudes, as a cap of tissue from which extend 
backwards two narrow horizontal wings, or *stabiHzers\ 
Several archegonia develop, in a row, across the upper side 
of the projecting cap, each with a short neck, a neck canal 
cell with two nuclei, and a ventral canal nucleus (Fig. 27M). 

Fig 27N illustrates a young sporeHng, still attached to the 
female prothallus within the megasporium, and shows the 
peculiar development of a 'column' (i), which separates the 
first leaf (2) and the stem (3) from the foot, which remains 
embedded in the prothallus. The early stages of segmenta- 
tion of the zygote are pecuHar and their morphological 
relationships are not fully estabUshed. At no stage is a root 
primordium distinguishable. 

If the relationships of the Marsileaceae are obscure, those 
of the Salviniales are even more so. The gradate origin of the 
sporangia within the sporocarp, the intercalary growth of 
the receptacle in Azolla and the vestigial oblique annulus 
have led to the suggestion that the group has affinities with 
the Hymenophyllaceae. However, this hardly seems accept- 
able, in view of the many extraordinary features that mark 
them off from all other ferns. 



General Conclusions 



In a book of this limited size it is impossible to describe 

in detail all the fossil plants that are known. Accordingly 

several major groups of vascular plants, many minor groups 

and a large number of genera have had to be omitted. Thus, 

there has been no mention of the Noeggerathiales, nor of the 

Pseudoborniales, on the grounds that they occupy isolated 

positions in the classification and throw almost no Hght at 

all on the evolution of modern plants. For details of these 

strange plants the reader is referred to textbooks of paleo- 
botany, i. s. i*. 22 

There are, also, several fossil genera of fronds and trunks 
of which no mention has been made, since they seem to 
stand midway between pteridophytes and gymnosperms 
(e.g. Aneurophyton, Eospermatopteris, Tetraxylopteris, Proto- 
pitys, Pitys, Archaeopteris, Callixylon and Archaeopitys), 
and might appropriately be described in a text-book of 
gymnosperms. However, there has been a recent suggestion^® 
that these plants, while indeed ancestral to the gymno- 
sperms, were, nevertheless, still at the level of pteridophytes 
in their mode of reproduction. Brief mention of them must, 
therefore, be made here. This suggestion arose out of the 
discovery, in Upper Devonian rocks near New York, of 
large fern-Hke fronds, known as Archaeopteris, actually in 
organic connection with Callixylon, a large tree whose mas- 
sive woody trunks were at least 20 m tall and more than i -5 m 
across. The fronds of one species oi Archaeopteris are known 

175 



176 THE MORPHOLOGY OF PTERIDOPHYTES 

to have had spores of two different sizes and hence cannot 
have borne seeds as well. The realization that trunks with 
this particular type of wood belonged to pteridophytes has 
come as a surprise to many morphologists, for it has been 
customary to think of them as gymnosperms. A new taxo- 
nomic group has been suggested (Progymnospermopsida) to 
contain the various genera listed above, that have affinities 
both with the Pteropsida and with the seed-bearing plants. 

While this group may indicate the direction in which the 
pteridophytes were evolving towards higher forms, there are 
unfortunately as yet no fossils linking them, in the reverse 
direction, with their possible ancestors. Discussions still take 
place as to whether pteridophytes evolved directly from 
Algae or from Bryophyta, and as to whether, in either case, 
they had a monophyletic or a polyphyletic origin. Until 
more fossils are known from the Ordovician, Cambrian and 
even the Pre-Cambrian, there would seem to be little hope of 
agreement on these matters. There are some, indeed, who 
doubt whether 'missing links' will ever be found. In the 
meantime, relying on what we know with certainty to have 
existed, we must guess at what their ancestors might have 
been like. 

Subjective processes of this kind have led to a number of 
theories of land-plant evolution, of which theTelome Theory 
has had the greatest number of adherents since it was first 
propounded by Zimmermann-^ in 1930. According to this 
theory, all vascular plants evolved from a very simple leafless 
ancestral type, Uke Rhynia, made up of sterile and fertile 
axes ('telomes'). In order to explain the wide diversity of 
organization found in later forms, a number of trends are 
supposed to have occurred, in varying degrees in the differ- 
ent taxonomic groups. These are represented diagrammati- 
cally in Fig. 28 (1-5) and are called respectively (i) plana- 
tion, (2) over-topping, (3) syngenesis, (4) reduction, (5) re- 
curving. 

Starting from a system of equal dichotomies in planes 



GENERAL CONCLUSIONS 



177 



successively at right angles (A), planation leads to a system 
of dichotomies in one plane (B). Overtopping is the result 
of unequal dichotomies, and tends to produce a main axis 
with lateral branches (C) ; the culmination of this process is 




Fig. 28 

The Telome Theory: 1, planation; 2, overtopping; 3, syngenesis; 
4, reduction; 5, re-curving, h-k, evolution in Sphenopsida; 
L-o, evolution in Pteropsida; p-s, evolution in Lycopsida. The 
Enation Theory : t-v, evolution of microphylls in Lycopsida 

(a-s, after Zimmermann; t-v, Bower) 

a monopodial system. Syngenesis results from the coales- 
cence of apical meristems. When they fuse to form a 
marginal meristem ('foliar syngenesis'), a lamina with veins 
develops (D) and the process is called 'webbing'. Zimmer- 
mann also visuahzes a second type of syngenesis ('axial 
syngenesis') in which several branches become absorbed 
into a single stout axis with a complex stelar anatomy (not 



178 THE MORPHOLOGY OF PTERIDOPHYTES 

shown in Fig. 28). While these three trends are in the direc- 
tion of progressive elaboration, the fourth is in the opposite 
direction ; reduction is supposed to have brought about the 
evolution of the simple unbranched microphyll of the 
Lycopsida. The fifth trend, re-curving, is found in several 
groups of plants, where the sporangiophore becomes re- 
flexed and the sporangium inverted, as in anatropous ovules. 

Figs. 28H-K illustrate the way in which the sporangio- 
phore might have evolved in the Sphenopsida. Here, re- 
curving and syngenesis are the chief trends, resulting in a 
peltate structure with reflexed sporangia. In the evolution of 
the leaf of the Sphenopsida, however, the chief trends have 
been planation, followed by reduction. Examples of inter- 
mediate types existed among fossil members of the group : 
Calamophyton, Hyenia, Eviostachya and Protocalamostachys 
represented stages in sporangiophore evolution, while 
Calamophyton and Asterocalamites provide stages in the 
evolution of leaves. The Telome Theory, therefore, gives a 
satisfactory explanation of the evolution of leaves, and of 
sporangiophores, in this group. However, the growth habit 
of the earliest members (e.g. Calamophyton, Protohyenia and 
Hyenia) was a long way from the theoretical ancestral type. 

In the Pteropsida (Figs. 28L-O), planation, overtopping 
and webbing have combined to produce the sterile and fertile 
fronds of modern ferns. The fossil record provides abundant 
examples of intermediate types of frond form (e.g. Pseudo- 
sporochnus, Stauropteris, Botryopteris) but, again, the 
growth habit of the earUest members was far removed from 
the ancestral type postulated by the Telome Theory (in fact, 
Cladoxylon was superficially very similar to Calamophyton). 

In the Lycopsida (Figs. 28P-S), the chief trend is supposed 
to have been reduction. The bifid tips of the sporophylls and 
leaves of Protolepidodendron may be brought forward in 
support of this suggestion, but otherwise the fossil record 
lacks good examples of intermediate types. The microphyll 
had almost completed its evolution by the time the group 



GENERAL CONCLUSIONS I79 

first appeared in the Cambrian {Aldanophyton) or the 
Silurian (Baragwanathia). 

The great appeal of the Telome Theory lies in its economy 
of hypotheses and in the way it allows the whole range of 
form of vascular plants to be seen in a single broad unified 
vista. Yet, to some botanists, this unifying influence is 
regarded as a dangerous over-simphfication, to be treated 
with great suspicion. It is in its appHcation to the Lycopsida 
that it is most open to criticism and, in our present state 
of knowledge, rightly so. The American palaeobotanist 
Andrews 2^ sums up his views in the following words: 
*Zimmermann's scheme for the pteropsids, or at least some 
pteropsids, has much supporting evidence; his concept for 
the articulates may be valid, but we are only on the verge of 
understanding the origins of this group ; his concept for the 
lycopsids is, so far as I am aware, purely hypothetical.' 

Figs. 28T-V illustrate the Enation Theory of Bower^ 
which suggests that microphylls are not homologous in any 
way with megaphylls. According to this theory, microphylls 
started as bulges from the surface of the stem, and then 
evolved into longer and longer projections, at first without 
any vascular supply, then with a leaf trace that stopped short 
in the cortex of the stem and, finally, with a vascular bundle 
running the whole length of the organ. The microphyll, 
therefore, has evolved by a gradual process of enlargement, 
rather than by progressive reduction, and for this theory the 
fossil record does provide some support : Psilophyton repre- 
sents the first stage in the process (Fig. 28T), Asteroxylon 
provides an example of the intermediate stage, where the 
leaf-trace stops short (Fig. 28U), while Drepanophycus 
represents a later stage with the leaf-trace entering the 
lateral appendage (Fig. 28V). 

Whether the Lycopsida evolved in this way, or in the 
manner suggested by Zimmermann, the starting point is, 
nevertheless, the same in both cases — a plant with naked 
forking axes — and it has been customary to quote Rhynia 



l8o THE MORPHOLOGY OF PTERIDOPHYTES 

and Horneophyton as examples of this type of plant. How- 
ever, they were certainly not the ancestors of pteridophytes. 
As Leclercq^^ emphasizes, they occurred much too late in 
the fossil record for this to be possible, and represent the 
last surviving examples of that particular growth form. As 
Andrews^* suggests, the emphasis that has been placed on 
Rhynia has drawn attention away from the great diversity 
of form that is now known in Silurian and Middle Devonian 
plants, and has led to an uncritical acceptance of the thesis 
that vascular plants are a monophyletic group. 

So far, these speculations as to the course of pteridophy te 
evolution have centred around the sporophyte, since it is 
this phase of the hfe cycle that is represented in the fossil 
record. Even more speculative is the evolution of gameto- 
phytes, concerning which there are the two diametrically 
opposed schools of thought referred to, near the end of 
Chapter 3, as 'Antithetic' and 'Homologous'. Mention was 
there made of abnormal gametophytes of Psilotum, contain- 
ing vascular tissues. The significance of this interesting dis- 
covery was somewhat diminished for a time, however, when 
it was shown that they were diploid ; but in relation to dis- 
cussions of antithesis and homology chromosome counts 
are, in a sense, *red-herrings'. This is made apparent by the 
phenomena of apogamy and apospory, cases of which have 
been recorded many times in pteridophytes since they were 
first observed in 1874. 

Apogamy is the development of a sporophyte directly 
from the gametophyte without the intermediate formation 
and fertihzation of gametes. The resulting sporophyte, 
therefore, has the same haploid chromosome count as the 
gametophyte. By 1939'^^ apogamy had been recorded among 
ferns, in Pteris, Dryopteris, Pellaea, and Trichomanes, where 
it is frequently preceded by the appearance of tracheids in 
the gametophyte. More recently ^^ it has been recorded in 
Thelypteris, Pteridium, Phyllitis and several species of Lyco- 
podium. In the case of Phyllitis, the haploid apogamous 



GENERAL CONCLUSIONS l8l 

sporophyte was successfully reared until it produced 
sporangia; however, as would be expected since it contained 
only one set of chromosomes, meiosis failed and no spores 
were produced. ^^ 

Apospory is, in a sense, the reverse process, being the 
production of gametophytes directly from sporophytes with- 
out the intermediate formation of spores. Thus, when 
detached pieces of fern fronds are placed on an agar surface 
they frequently develop directly into gametophytes of 
normal shape and form. In such cases, the gametophyte has 
the same diploid chromosome count as the sporophyte. So 
numerous are the recorded instances of this phenomenon 
that BelP^ suggests that it must be general among ferns; 
yet the exact conditions under which it happens cannot yet 
be specified. 

As to the causes of apogamy, several theories have been 
put forward, but the final word has certainly not been said 
on this fascinating problem. In many cases, ageing of the 
prothallus seems to be an important factor. Recent work in 
America^^ on Osmunda, Adiantum and Pteridium has, how- 
ever, demonstrated that apogamy can be induced by grow- 
ing the prothaUi on an agar culture medium rich in glucose. 
Clearly, therefore, under these highly artificial circum- 
stances, the external environment can be an important 
factor. That this might be so had been suspected for a long 
time, since otherwise it was difficult to understand why a 
diploid zygote developing inside a fertilized archegonium 
should give rise to a sporophyte, while a diploid cell 
developing by apospory should give rise to a gametophyte. 
Confirmation of the view that the internal environment of 
the archegonium exerts an important formative influence on 
the nature of the embryo has recently come from experi- 
ments in which young embryos of Todea were dissected 
from the archegonium and grown on an artificial medium. ^^^ 
It was found that those removed before the first division of 
the zygote developed into flat thalloid structures, whereas 



l82 THE MORPHOLOGY OF PTERIDOPHYTES 

those removed in later stages of development grew into 
normal sporophytes. Whether the environment is entirely 
responsible, however, for the normal regular alternation of 
generations has been questioned. BelP'^ suggests that there 
must be some internal factor at work and looks upon 
gametophyte and sporophyte as two levels of complexity, 
reflecting different states of the cytoplasm, which can be 
accounted for in terms of cell chemistry. This interesting 
hypothesis should stimulate further research into the causes 
of alternation of generations in hving plants. 

The present position, then, seems to be that there is no 
fundamental distinction between gametophytes and sporo- 
phytes, since they can be induced to change from one to the 
other in either direction. They are 'homologous', as far as 
can be judged from living plants, and one is led to speculate, 
therefore, that they were probably ahke in form and structure 
in the earliest ancestors of land plants. Merker's suggestion,^^ 
already mentioned in Chapter 2, that the horizontal axes of 
the Rhyniaceae were gametophytes, instead of sporophytic 
rhizomes, is of enhanced interest, therefore, because if con- 
firmed it will provide the only kind of evidence which can 
really settle the controversy. As with most problems of 
macro-evolution, it is the palaeobotanist who has the key 
within his reach. 



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84 Wand, A., 1914. Flora, 106, 237-263. (Apical meristems in 

Selaginella.) 

85 Ward, M., 1954. Phytomorph., 4, 1-17. (Embryology in the 

fern, Phlebodium aurem.) 

86 West, R. G., 1953- New Phytol, 52, 267-272. {Azolla in 

interglacial deposits.) 



l88 BIBLIOGRAPHY 

87 Whittier, D. P., and Steeves, T. A., i960. Can. J. Bot., 38, 

925-930. (Apogamy in ferns brought about by glucose.) 

88 Williams, S., 1933. Trans, roy. Soc. Edinb., 57, 711-737. 

(Regeneration in Lycopodium Selago.) 

89 Williamson, W. C. and Scott, D. H., 1894. Phil. Trans. 

B 185, 863-959. {Palaeostachya vera.) 

90 Zimmermann, W., 1952. The Palaeobotanist, i, 456-470. 

(The main results of the Telome Theory.) 



Index 



Page numbers in italic refer to illustrations 



Acitheca, 128, 129 
acrostichoid condition, 150 
AcrosHchum, 164 
'Adder's tongue' fern, 135, 137 
Adiantaceae, 166 
Adiantum, 153, 156, 167, 181 
advancement index, 151 
Aldanophyton, 51, 53, 179 
Alsophila, 161 
Alsophilites, 161 

alternation of generations, 13, 182 
Anemia, 153, 154 
Angiopteridaceae, 127 
Angiopteris, 128, 130 
angle meristem, 84, 87 
Ankyropteris , 118, 121 
Annular ia, 102, 104 
annulus, 144, 149, 153 
Anogtamma, 167 
antheridium, 13 
antherozoid, 13 
antithetic theory, 48, 180 
aphlebiae, 117, 118, 123 
apical meristem, 21, 57, 87 
apogamy, 180 
apospory, 181 
Archaeocalamites, loi, 102 
Archaeopteris, 175 
archegonium, 13, 45 
archesporial cells, 60 
Arthropitys, 104 
Arthrostigma, 52 

Articulatae (= Sphenopsida), 94 
Asplenioideae, 165 
Aspleniuni, 156, 165 
Asterocalamitaceae, loi 
Asterocalamites, loi, 102, 178 
Asterophyllites, 102, 104 
Asterotheca, 127, 128 
Asterothecaceae, 127 
Asteroxylaceae, 28 
Asteroxylon, 34, 35, 179 
Athyrioideae, 165 



Athyrium, 156, 165 
Austroclepis, 120 
Azolla, 171, 173 
Azollaceae, 171 

Baragwanathia, 51, 179 
Blechnoideae, 166 
Blechnum, 156, 166 
Bothrodendraceae, 68 
Bothrodendron, 74 
Botrychioxylon, 122 
Botrychium, 135, 137 
Botryopteridaceae, 124 
Botryopteris, 124, 125, 178 
Bowmanites, g6, 99 
bulbils, 54 

CALAMITACEAE, lOI 

Calamitales, loi 

Calamites, 102, 103 

Calamophytaceae, 94 

Calamophyton, 95, g6, 178 

Calamostachys, 102, 105 

Callixylon, 175 

cambium, 22, 69, 76, 83, 95, 103, 

117, 136 
Camptopteris , 160 
Cardiomanes, 157 
carinal canals, 98, loi, 104, 109 
casts, 22, 104 
Ceratopteris, 167 
Cheilanthes, 167 
Cheirostrobaceae, 94 
Cheirostrobus, 96, 99 
Christens enia, 128, 130 
Christenseniaceae, 127 
Cibotium, 158 

circinate vernation, 34, 35, 131, 143 
circummeduUary strands, 73 
Cladoxylaceae, 115 
Cladoxylales, 116 
Cladoxylon, 116, 118, 178 
classification, 27 



189 



190 INDEX 



Clathropteris, i6o 
Clepsydropsis, 117, 118 
club mosses, 53 
Coenopteridales, 119 
compressions, 23 
Coniopteris, 158 
Cooksonia, 31, 32 
cork cambium, 22 
cover cell, 45, 47 
Cyathea, 156, 161 
Cyatheaceae, 160 
CyHndrostachya, 83 

Danaea, 128, 133 
Danaeaceae, 127 
Davallia, 163 
Davallioideae, 163 
Dennstaedtia, 156, 162 
Dennstaedtiaceae, 162 
Dennstaedtioideae, 162 
dermal appendages, 131, 150 
diaphragm, 84, 90 
Dicksonia, 156, 158 
Dicksopiaceae, 158 
Dictyophyllum, 160 
dictyostele, 18, ig 
dictyoxylic stele, 142 
dimorphism, 164, 167 
Dineuron, 121 
dioecism, 15, 112 
diploid, 13 
Dtplolabis, 120 
Dipteridaceae, 160 
Dipteris, 153, 160 
Drepanophycaceae, 50 
Drepanophycus, 51, 52, 179 
Dryopteridoideae, 164 
Dryopteris, 156, 164, 180 

Elaphoglossum, 165 
elaters, 102, 112 
enation theory, 177, 179 
endarch xylem, 21 
endoscopic embryology, 81 
endosporic development, 14, 73, 80 
Eoangiopteris, 128, 129 
epibasal, 46, 63 
Equisetaceae, 94 
Equisetales, 106 
Equisetites, 113 
Equisetum, 102, 106 
Etapteris, 118, 122 
Eucalamites, 102, 104 
Eusporangiatae, 127 
eusporangiate development, 16 
Eviostachya, 96, 99, 178 



exarch xylem, 21 

exoscopic embryology, 47, 113, 140 

FIBONACCI SERIES, 59, 70 

filamentous prothalli, 154, 158 

Filicales, 145 

filmy ferns, 155 

foot, 46, 61, 63, 90, 113, 135, 139, 

149 
foramen, 79 
form genera, 74 
fossilization, 22 

GAMETOPHYTE, 1 3 

geological periods, 24, 25 
Gleichenia, 153, 154, 136 
Gleicheniaceae, 154 
Gleichenites, 146 
glochidia, 172, 173 
glossopodium, 66, 88 
Gradatae, 150 
ground pines, 56 

HAPLOiD, 13 

Helminthostachys, 135 

Hemitelia, 161 

Heterophyllum, 83 

heterospory, 14, 17, 73, 80, 89, 100, 

105, 112, 123, 170, 172 
Hierogramtna, 117, 118 
Homoeophyllum, 83 
homologous theory, 48, 180 
homospory, 13 
Hornea, see Horneophyton 
Horneophyton, 30, 31, 180 
horsetails, 106 
Hydropterideae, 168 
Hyenia, 95, g6, 178 
Hyeniaceae, 94 
Hyeniales, 95 
Hymenophyllaceae, 155 
Hymenophyllum, 153, 155 
hypobasal, 46, 63 

iNDusiUM, 156, 158 
Isoetaceae, 50 
Isoetales, 76 
Isoetes, 76, 78 

JACKET INITIAL, 46 

Klukia, 146 

LEAF GAPS, 18 

Lepidocarpon, 71, 73 
Lepidodendraceae, 68 
Lepidodendrales, 68 



INDEX 



191 



Lepidodendron, 67, 68, yi 
Lepidophloios, 67, 69 
Lepidophylluni, 70 
Lepidostrobus, 71, 72 
Leptopteris, 143, 145 
Leptosporangiatae, 145, 147 
leptosporangiate development, 16, 

145, 147 
Ligulatae, 66 
ligule, 66, 71, 78 
Litostrobus, 100 
Lornaria, 166 
Lomariopsidoideae, 165 
Lycopodiaceae, 50 
Lycopodiales, 53 
Lycopodites, 64 
Lycopodium, 53, 55, 61, 67 
Lycopsida, 50, 177 
Lygodiiim, 152, 153 

Marattia, 12S, 130 

Marattiaceae, 127 

Marattiales, 127 

Marsilea, 168, i6g 

Marsileaceae, 168 

Marsileales, 168 

massnlae, 172, 173 

Matonia, 153, 156, i59 

Matoniaceae, 159 

Matonidium, 159 

Matteuccia, 156, 164 

megaphylls, 18, 114 

megaspores, 14, 73, 80, 90, 124, 170, 

172 
meiosis, 13 
meristeles, 20 
mesarch xylem, 21 
M esocalamites , 104 
Metaclepsydropsis, 118, 120 
metaxylem, 21 
Microlepia, 156, 163 
microphylls, 18, 179 
microspores, 14, 73, 80, 89, 170, 172 
Mixtae, 150 
Mohria, 153 
monoecism, 15 
moonwort, 135 
morphology, 11 
mummifications, 23 
mycorrhiza, 32, 39, 43, 45, 62, 87, 

133, 134, 136, 139, 155 

NECK CANAL CELL, 46, 62 

neck cell, 46 
neoteny, 65 
Nephrolepis, 156, 165 



OLEANDROIDEAE, 163 

Oligocarpia, 146 
Oligomacrosporangiatae, 83 
Onoclea, 164 
Onocleoideae, 164 
Ophioglossaceae, 127 
Ophioglossales, 135 
Ophioglossum, 135, 137 
Osmunda, 142, 145, 181 
Osmundaceae, 141 
Osmundales, 141 
Osmundidae, 141 
Osmundites, 141 
out-breeding, 16 
overtopping, J77 

Palaeostachya, 102, 105 

parichnos strands, 70 

Pecopteris, 127, 128 

Pellaea, 167, 180 

perforated steles, 20 

periderm, 22, 69 

peripheral loops, 118, 120 

petrifactions, 23 

Phanerosorus, 156, 159 

phlobaphene, 40, 44 

phyletic slide, in Palaeostachya, 106 

in ferns, 163 
Phyllitis, 156, 166, 180 
Phylloglossum, 61, 65 
phyllophore, 120 
phyllotaxy, 59, 70, 77, 120 
phylogeny, 12 
Pilularia, 168, i6g 
Pilulariaceae, 168 
planation, 177 
Platy cerium, 167 
Pleiomacrosporangiatae, 83 
Pleuromeia, 75, 78 
Pleuromeiaceae, 68 
polycyclic steles, 19, 21, 85, 159 
polyploidy, 49, 66 
Polypodiaceae, {sensu lato), 162 

[sensu stricto), 167 
Polypodium, 167 
polystely, 21, 84, 85, 116, 118 
Polystichum, 156, 165 
Pothocites, 103 

primary spermatogenous cell, 46 
primitive characters in Filicales, 149 
Primofilices, 115 
prismatic tissue, 77 
prothallial cell, 80, 90 
prothallus, 14 
Protocalamites, loi, 102 
Protocalamostachys, 102, 103, 178 
protocorm, 64 



192 INDEX 

Protohyenia, 95, g6, 178 
Protohyeniaceae, 94 
Protolepidodendraceae, 50 
Protolepidodendrales, 51 
Protolepidodendron, 51, 52, 178 
protophyll, 64 
protostele, 18, jp 
protoxylem, 21 
Psaronius, 129 
Pseudosporochnaceae, 115 
Pseudosporochnus, 117, 118, 178 
Psilophytaceae, 28 
Psilophy tales, 28 
Psilophyton, 33, J5, i79 
Psilophytopsida, 28 
Psilotaceae, 38 
Psilotales, 38 
Psilotopsida, 38 
Psilotum, 38, 41, 47, 180 
Pteridium, 156, 163, 180 
Pteridoideae, 163 
Pteris, 156, 164, 180 
Pteropsida, 114 

QUiLLWORT, 76 

RECAPITULATION, doctrine of, 57 
recurving, 176, J77 
reduction, J77, 178 
Regnellidium, 168 
resvirrection plants, 82 
Rhacophyton, 121 
rhizophores, 84, 87 
Rhopalostachya, 54 
Rhynia, 29, 31, 179 
Rhyniaceae, 28, 182 
royal fern, 142 

Salvinia, 172, 173 
Salviniaceae, 171 
Salviniales, 171 
Schizaea, 153 
Schizaeaceae, 152 
Scolecopteris, 128, 129 
secretory tissue, 70 
Selaginella, 82, 84 
Selaginellaceae, 50 
Selaginellales, 82 
Selaginellites, 91 
Senftenbergia, 146 
Sigillaria, 71, 73 
Sigillariaceae, 68 
Simplices, 150 
siphonostele, 20 
solenostele, 18, jp 
solenoxylic stele, 136 
SphenophyUaceae, 94 



Sphenophyllales, 97 

Sphenophyllostachys, g6, 99 

Sphenophyllum, g6, 97 

Sphenopsida, 94 

sporangia, 16 

sporocarps, 168, i6g, 171, 173 

sporophore, 170 

sporophyll, 17, 52, 54, 73, 75, 81 

sporophyll theory, 17 

sporophyte, 13 

'Stag's horn' fern, 167 

Stauropteridaceae, 116 

Stauropteris, 123, 123, 178 

steles, 18, Jp 

Stenochlaena, 164 

Stigmaria, 71, 74 

stomium, 124, 126, 144 

strobilus, 56 

Stylites, 78, 81 

Stylocalamites, 104 

suspensor, 61, 63, 134, 140 

Syncardia, 117, 118 

syngenesis, 177 

TAPETUM, 42, 60, 133, 144, 147 

telome theory, 176 
teratology, 44 
Thamnopteris, 141 
Thelyptens, 180 
Tmesipteridaceae, 38 
Tmesipteris, 41, 42, 47 
Todea, 142, 145, 181 
tree ferns, 158, 160 
Trichomanes, 156, 157, 180 
Triletes, 91 
Tuhicaulis, 123 

UNLIMITED GROWTH IN FERN FRONDS, 
152, 154, J56, 159 

Urostachya, 54 

VALLECULAR CANALS, Io8 

velum, 78, 79 
ventral canal cell, 46 
vessels, 86, 109 

WATER FERNS, l68 

Woodwardia, 166 

Yarravia, 31, 32 

Zalesskya, 141 
ZosterophyUaceae, 28 
Zosterophyllum, 31, 33 
Zygopteridaceae, 115 
Zygopteris, 122 
zygote, 13